Saccharomyces Kluyveri Research Articles

The spatiotemporal program of replication in the genome of Lachancea kluyveri.

We generated a genome-wide replication profile in the genome of Lachancea kluyveri and assessed the relationship between replication and base composition. This species diverged from Saccharomyces cerevisiae before the ancestral whole genome duplication. The genome comprises eight chromosomes among which a chromosomal arm of 1 Mb has a G + C-content much higher than the rest of the genome. We identified 252 active replication origins in L. kluyveri and found considerable divergence in origin location with S. cerevisiae and with Lachancea waltii. Although some global features of S. cerevisiae replication are conserved: Centromeres replicate early, whereas telomeres replicate late, we found that replication origins both in L. kluyveri and L. waltii do not behave as evolutionary fragile sites. In L. kluyveri, replication timing along chromosomes alternates between regions of early and late activating origins, except for the 1 Mb GC-rich chromosomal arm. This chromosomal arm contains an origin consensus motif different from other chromosomes and is replicated early during S-phase. We showed that precocious replication results from the specific absence of late firing origins in this chromosomal arm. In addition, we found a correlation between GC-content and distance from replication origins as well as a lack of replication-associated compositional skew between leading and lagging strands specifically in this GC-rich chromosomal arm. These findings suggest that the unusual base composition in the genome of L. kluyveri could be linked to replication.

Comparative Mitochondrial Genomics within and among Yeast Species of the Lachancea Genus

Yeasts are leading model organisms for mitochondrial genome studies. The explosion of complete sequence of yeast mitochondrial (mt) genomes revealed a wide diversity of organization and structure between species. Recently, genome-wide polymorphism survey on the mt genome of isolates of a single species, Lachancea kluyveri, was also performed. To compare the mitochondrial genome evolution at two hierarchical levels: within and among closely related species, we focused on five species of the Lachancea genus, which are close relatives of L. kluyveri. Hence, we sequenced the complete mt genome of L. dasiensis, L. nothofagi, L. mirantina, L. fantastica and L. meyersii. The phylogeny of the Lachancea genus was explored using these data. Analysis of intra- and interspecific variability across the whole Lachancea genus led to the same conclusions regarding the mitochondrial genome evolution. These genomes exhibit a similar architecture and are completely syntenic. Nevertheless, genome sizes vary considerably because of the variations of the intergenic regions and the intron content, contributing to mitochondrial genome plasticity. The high variability of the intergenic regions stands in contrast to the high level of similarity of protein sequences. Quantification of the selective constraints clearly revealed that most of the mitochondrial genes are under purifying selection in the whole genus.

Mitochondrial Genome Evolution in a Single Protoploid Yeast Species

Mitochondria are organelles, which play a key role in some essential functions, including respiration, metabolite biosynthesis, ion homeostasis, and apoptosis. The vast numbers of mitochondrial DNA (mtDNA) sequences of various yeast species, which have recently been published, have also helped to elucidate the structural diversity of these genomes. Although a large corpus of data are now available on the diversity of yeast species, little is known so far about the mtDNA diversity in single yeast species. To study the genetic variations occurring in the mtDNA of wild yeast isolates, we performed a genome-wide polymorphism survey on the mtDNA of 18 Lachancea kluyveri (formerly Saccharomyces kluyveri) strains. We determined the complete mt genome sequences of strains isolated from various geographical locations (in North America, Asia, and Europe) and ecological niches (Drosophila, tree exudates, soil). The mt genome of the NCYC 543 reference strain is 51,525 bp long. It contains the same core of genes as Lachancea thermotolerans, the nearest relative to L. kluyveri. To explore the mt genome variations in a single yeast species, we compared the mtDNAs of the 18 isolates. The phylogeny and population structure of L. kluyveri provide clear-cut evidence for the existence of well-defined geographically isolated lineages. Although these genomes are completely syntenic, their size and the intron content were found to vary among the isolates studied. These genomes are highly polymorphic, showing an average diversity of 28.5 SNPs/kb and 6.6 indels/kb. Analysis of the SNP and indel patterns showed the existence of a particularly high overall level of polymorphism in the intergenic regions. The dN/dS ratios obtained are consistent with purifying selection in all these genes, with the noteworthy exception of the VAR1 gene, which gave a very high ratio. These data suggest that the intergenic regions have evolved very fast in yeast mitochondrial genomes.

A New Expression Vector for Production of Enzymes in the Yeast Saccharomyces (Lachancea) Kluyveri

We overexpressed and purified enzymes involved in the pyrimidine catabolic pathway in the yeast Saccharomyces (Lachancea) kluyveri. A new vector was therefore designed, providing the first specific expression system in Saccharomyces kluyveri. The URC1 gene was overexpressed and a soluble protein obtained and successfully purified using the C-terminally added His-tag. Our system will be used for further studies of the structure and function of the enzymes belonging to the URC pyrimidine degradation pathway.

Novel features of ARS selection in budding yeast Lachancea kluyveri

BackgroundThe characterization of DNA replication origins in yeast has shed much light on the mechanisms of initiation of DNA replication. However, very little is known about the evolution of origins or the evolution of mechanisms through which origins are recognized by the initiation machinery. This lack of understanding is largely due to the vast evolutionary distances between model organisms in which origins have been examined.ResultsIn this study we have isolated and characterized autonomously replicating sequences (ARSs) in Lachancea kluyveri - a pre-whole genome duplication (WGD) budding yeast. Through a combination of experimental work and rigorous computational analysis, we show that L. kluyveri ARSs require a sequence that is similar but much longer than the ARS Consensus Sequence well defined in Saccharomyces cerevisiae. Moreover, compared with S. cerevisiae and K. lactis, the replication licensing machinery in L. kluyveri seems more tolerant to variations in the ARS sequence composition. It is able to initiate replication from almost all S. cerevisiae ARSs tested and most Kluyveromyces lactis ARSs. In contrast, only about half of the L. kluyveri ARSs function in S. cerevisiae and less than 10% function in K. lactis.ConclusionsOur findings demonstrate a replication initiation system with novel features and underscore the functional diversity within the budding yeasts. Furthermore, we have developed new approaches for analyzing biologically functional DNA sequences with ill-defined motifs.

Differences in the Number of Intrinsically Disordered Regions between Yeast Duplicated Proteins, and Their Relationship with Functional Divergence

BackgroundIntrinsically disordered regions are enriched in short interaction motifs that play a critical role in many protein-protein interactions. Since new short interaction motifs may easily evolve, they have the potential to rapidly change protein interactions and cellular signaling. In this work we examined the dynamics of gain and loss of intrinsically disordered regions in duplicated proteins to inspect if changes after genome duplication can create functional divergence. For this purpose we used Saccharomyces cerevisiae and the outgroup species Lachancea kluyveri.Principal FindingsWe find that genes duplicated as part of a genome duplication (ohnologs) are significantly more intrinsically disordered than singletons (p<2.2e-16, Wilcoxon), reflecting a preference for retaining intrinsically disordered proteins in duplicate. In addition, there have been marked changes in the extent of intrinsic disorder following duplication. A large number of duplicated genes have more intrinsic disorder than their L. kluyveri ortholog (29% for duplicates versus 25% for singletons) and an even greater number have less intrinsic disorder than the L. kluyveri ortholog (37% for duplicates versus 25% for singletons). Finally, we show that the number of physical interactions is significantly greater in the more intrinsically disordered ohnolog of a pair (p = 0.003, Wilcoxon).ConclusionThis work shows that intrinsic disorder gain and loss in a protein is a mechanism by which a genome can also diverge and innovate. The higher number of interactors for proteins that have gained intrinsic disorder compared with their duplicates may reflect the acquisition of new interaction partners or new functional roles.

Mechanisms of Chromosome Number Evolution in Yeast

The whole-genome duplication (WGD) that occurred during yeast evolution changed the basal number of chromosomes from 8 to 16. However, the number of chromosomes in post-WGD species now ranges between 10 and 16, and the number in non-WGD species (Zygosaccharomyces, Kluyveromyces, Lachancea, and Ashbya) ranges between 6 and 8. To study the mechanism by which chromosome number changes, we traced the ancestry of centromeres and telomeres in each species. We observe only two mechanisms by which the number of chromosomes has decreased, as indicated by the loss of a centromere. The most frequent mechanism, seen 8 times, is telomere-to-telomere fusion between two chromosomes with the concomitant death of one centromere. The other mechanism, seen once, involves the breakage of a chromosome at its centromere, followed by the fusion of the two arms to the telomeres of two other chromosomes. The only mechanism by which chromosome number has increased in these species is WGD. Translocations and inversions have cycled telomere locations, internalizing some previously telomeric genes and creating novel telomeric locations. Comparison of centromere structures shows that the length of the CDEII region is variable between species but uniform within species. We trace the complete rearrangement history of the Lachancea kluyveri genome since its common ancestor with Saccharomyces and propose that its exceptionally low level of rearrangement is a consequence of the loss of the non-homologous end joining (NHEJ) DNA repair pathway in this species.

Microbiology and Nutritional Value of Selroti, an Ethnic Fermented Cereal Food of the Himalayas

Selroti is an ethnic fermented rice food of the Himalayas. A total of 125 samples of selroti batters were collected from different villages and markets of the Himalayas. The microbial population of selroti batters showed that lactic acid bacteria (LAB) were present in viable numbers above 108 cfu/g, followed by yeasts at 105 cfu/g. LAB Leuconostoc mesenteroides, Enterococcus faecium, Pediococcus pentosaceus and Lactobacillus curvatus and yeasts Saccharomyces cerevisiae, Saccharomyces kluyveri, Debaryomyces hansenii, Pichia burtonii, and Zygosaccharomyces rouxii were identified. The most prevalent LAB and yeasts in selroti batters were Leuc. mesenteroids (42.9%) and S. cerevisiae (35.6%). Molds and pathogenic bacteria were not detected. It was observed that seasons affect the development and prevalence of microorganisms in the fermented batters. LAB and yeast strains were screened for their acidifying and coagulating capacity, and it was found that most of the LAB strains acidified with lowering of pH up to 4.3. These strains showed a wide spectrum of enzymatic profiles in commercial API-zym kits. All strains of LAB showed antimicrobial activities under the applied condition. The nutritional value of fermented batters was found to be increased. This is the first report on selroti concerning its microbiology and nutritional value.

Characterization of a New Multigene Family Encoding Isomaltases in the Yeast Saccharomyces cerevisiae, the IMA Family

It has been known for a long time that the yeast Saccharomyces cerevisiae can assimilate alpha-methylglucopyranoside and isomaltose. We here report the identification of 5 genes (YGR287c, YIL172c, YJL216c, YJL221c and YOL157c), which, similar to the SUCx, MALx, or HXTx multigene families, are located in the subtelomeric regions of different chromosomes. They share high nucleotide sequence identities between themselves (66-100%) and with the MALx2 genes (63-74%). Comparison of their amino acid sequences underlined a substitution of threonine by valine in region II, one of the four highly conserved regions of the alpha-glucosidase family. This change was previously shown to be sufficient to discriminate alpha-1,4- to alpha-1,6-glucosidase activity in YGR287c (Yamamoto, K., Nakayama, A., Yamamoto, Y., and Tabata, S. (2004) Eur. J. Biochem. 271, 3414-3420). We showed that each of these five genes encodes a protein with alpha-glucosidase activity on isomaltose, and we therefore renamed these genes IMA1 to IMA5 for IsoMAltase. Our results also illustrated that sequence polymorphisms among this family led to interesting variability of gene expression patterns and of catalytic efficiencies on different substrates, which altogether should account for the absence of functional redundancy for growth on isomaltose. Indeed, deletion studies revealed that IMA1/YGR287c encodes the major isomaltase and that growth on isomaltose required the presence of AGT1, which encodes an alpha-glucoside transporter. Expressions of IMA1 and IMA5/YJL216c were strongly induced by maltose, isomaltose, and alpha-methylglucopyranoside, in accordance with their regulation by the Malx3p-transcription system. The physiological relevance of this IMAx multigene family in S. cerevisiae is discussed.

Intestinal absorption of dietary maize glucosylceramide in lymphatic duct cannulated rats

Sphingolipids are ubiquitous in all eukaryotic organisms. Various physiological functions of dietary sphingolipids, such as preventing colon cancer and improving the skin barrier function, have been recently reported. One of the common sphingolipids used as a foodstuff is glucosylceramide from plant sources, which is composed of sphingoid bases distinct from those of mammals. However, the fate of dietary sphingolipids derived from plants is still not understood. In this study, we investigated the absorption of maize glucosylceramide in the rat intestine using a lipid absorption assay of lymph from the thoracic duct. The free and complex forms of trans-4,cis-8-sphingadienine, the predominant sphingoid base of maize glucosylceramide, were found in the lymph after administration of maize glucosylceramide. This plant type of sphingoid base was detected in the ceramide fraction and N-palmitoyl-4,8-sphingadienine (C16:0-d18:2) and N-tricosanoyl-4,8-sphingadienine (C23:0-d18:2) were identified by LC-MS/MS. The cumulative recovery of 4t,8c-sphingadienine in the lymph was very low. These results indicate that dietary glucosylceramide originating from higher plants is slightly absorbed in the intestine and is incorporated into ceramide structures in the intestinal cells. However, it appears that the intact form of sphingoid bases is not reutilized well in the tissues.

Amino Acid Signaling in Yeast: Post-genome Duplication Divergence of the Stp1 and Stp2 Transcription Factors

When yeast cells detect external amino acids via their permease-like Ssy1 sensor, the cytosolic precursor forms of Stp1 and Stp2 transcription factors are activated by endoproteolytic removal of their N-terminal domains, a reaction catalyzed by the Ssy5 endoprotease. The processed Stp factors then migrate into the nucleus, where they activate transcription of several amino acid permease genes including AGP1. We report here that the STP1 and STP2 genes most likely derive from the whole genome duplication that occurred in a yeast ancestor. Although Stp1 and Stp2 have been considered redundant, we provide evidence that they functionally diverged during evolution. Stp2 is the only factor processed when amino acids are present at low concentration, and the transcriptional activation of AGP1 promoted by Stp2 is moderate. Furthermore, only Stp2 can sustain Agp1-dependent utilization of amino acids at low concentration. In contrast, Stp1 is only processed when amino acids are present at high concentration, and it promotes higher level transcriptional activation of AGP1. Domain swapping experiments show that the N-terminal domains of Stp1 and Stp2 are responsible for these proteins being cleaved at different amino acid concentrations. Last, induction of the DIP5 permease gene by amino acids depends on Stp2 but not Stp1. We propose that post-whole genome duplication co-conservation of the STP1 and STP2 genes was favored by functional divergence of their products, likely conferring to cells an increased ability to adapt to various amino acid supply conditions.

Insertion of horizontally transferred genes within conserved syntenic regions of yeast genomes.

Horizontal gene transfer has been occasionally mentioned in eukaryotic genomes, but such events appear much less numerous than in prokaryotes, where they play important functional and evolutionary roles. In yeasts, few independent cases have been described, some of which corresponding to major metabolic functions, but no systematic screening of horizontally transferred genes has been attempted so far. Taking advantage of the synteny conservation among five newly sequenced and annotated genomes of Saccharomycetaceae, we carried out a systematic search for HGT candidates amidst genes present in only one species within conserved synteny blocks. Out of 255 species-specific genes, we discovered 11 candidates for HGT, based on their similarity with bacterial proteins and on reconstructed phylogenies. This corresponds to a minimum of six transfer events because some horizontally acquired genes appear to rapidly duplicate in yeast genomes (e.g. YwqG genes in Kluyveromyces thermotolerans and serine recombinase genes of the IS607 family in Saccharomyces kluyveri). We show that the resulting copies are submitted to a strong functional selective pressure. The mechanisms of DNA transfer and integration are discussed, in relation with the generally small size of HGT candidates. Our results on a limited set of species expand by 50% the number of previously published HGT cases in hemiascomycetous yeasts, suggesting that this type of event is more frequent than usually thought. Our restrictive method does not exclude the possibility that additional HGT events exist. Actually, ancestral events common to several yeast species must have been overlooked, and the absence of homologs in present databases leaves open the question of the origin of the 244 remaining species-specific genes inserted within conserved synteny blocks.

Unusual composition of a yeast chromosome arm is associated with its delayed replication

The 11.3-Mb genome of the yeast Lachancea (Saccharomyces) kluyveri displays an intriguing compositional heterogeneity: a region of approximately 1 Mb, covering almost the whole left arm of chromosome C (C-left), has an average GC content of 52.9%, which is significantly higher than the 40.4% global GC content of the rest of the genome. This region contains the MAT locus, which remains normal in composition. The excess of GC base pairs affects both coding and noncoding sequences, and thus is not due to selective pressure acting on protein sequences. It leads to a strong codon usage bias and alters the amino acid composition of the 457 proteins encoded on C-left that do not show obvious bias for functional categories, or the presence of paralogs or orthologs of essential genes of Saccharomyces cerevisiae. They share significant synteny conservation with other species of the Saccharomycetaceae, and phylogenetic analysis indicates that C-left originates from a Lachancea species. In contrast, there is a complete absence of transposable elements in C-left, whereas 18 elements per megabase are distributed across the rest of the genome. Comparative hybridization of synchronized cells using high-density genome arrays reveals that C-left is replicated later during S phase than the rest of the genome. Two possible primary causes of this major compositional heterogeneity are discussed: an ancient hybridization of two related species with very distinct GC composition, or an intrinsic mechanism, possibly associated with the loss of the silent cassettes from C-left that progressively increased the GC content and generated the delayed replication of this chromosomal arm.

Comparative genomics of protoploid Saccharomycetaceae

Our knowledge of yeast genomes remains largely dominated by the extensive studies on Saccharomyces cerevisiae and the consequences of its ancestral duplication, leaving the evolution of the entire class of hemiascomycetes only partly explored. We concentrate here on five species of Saccharomycetaceae, a large subdivision of hemiascomycetes, that we call "protoploid" because they diverged from the S. cerevisiae lineage prior to its genome duplication. We determined the complete genome sequences of three of these species: Kluyveromyces (Lachancea) thermotolerans and Saccharomyces (Lachancea) kluyveri (two members of the newly described Lachancea clade), and Zygosaccharomyces rouxii. We included in our comparisons the previously available sequences of Kluyveromyces lactis and Ashbya (Eremothecium) gossypii. Despite their broad evolutionary range and significant individual variations in each lineage, the five protoploid Saccharomycetaceae share a core repertoire of approximately 3300 protein families and a high degree of conserved synteny. Synteny blocks were used to define gene orthology and to infer ancestors. Far from representing minimal genomes without redundancy, the five protoploid yeasts contain numerous copies of paralogous genes, either dispersed or in tandem arrays, that, altogether, constitute a third of each genome. Ancient, conserved paralogs as well as novel, lineage-specific paralogs were identified.

The oxygen level determines the fermentation pattern inKluyveromyces lactis

Yeasts belonging to the lineage that underwent whole-genome duplication (WGD) possess a good fermentative potential and can proliferate in the absence of oxygen. In this study, we analyzed the pre-WGD yeast Kluyveromyces lactis and its ability to grow under oxygen-limited conditions. Under these conditions, K. lactis starts to increase the glucose metabolism and accumulates ethanol and glycerol. However, under more limited conditions, the fermentative metabolism decreases, causing a slow growth rate. In contrast, Saccharomyces cerevisiae and Saccharomyces kluyveri in anaerobiosis exhibit almost the same growth rate as in aerobiosis. In this work, we showed that in K. lactis, under oxygen-limited conditions, a decreased expression of RAG1 occurred. The activity of glucose-6-phosphate dehydrogenase also decreased, likely causing a reduced flux in the pentose phosphate pathway. Comparison of related and characterized yeasts suggests that the behavior observed in K. lactis could reflect the lack of an efficient mechanism to maintain a high glycolytic flux and to balance the redox homeostasis under hypoxic conditions. This could be a consequence of a recent specialization of K. lactis toward living in a niche where the ethanol accumulation at high oxygen concentrations and the ability to survive at a low oxygen concentration do not represent an advantage.

Substrate Specificity and Regioselectivity of Δ12 and ω3 Fatty Acid Desaturases fromSaccharomyces kluyveri

Delta12 and omega3 fatty acid desaturases are key enzymes in the synthesis of polyunsaturated fatty acids (PUFAs), which are important constituents of membrane glycerolipids and also precursors to signaling molecules in many organisms. In this study, we determined the substrate specificity and regioselectivity of the Delta12 and omega3 fatty acid desaturases from Saccharomyces kluyveri (Sk-FAD2 and Sk-FAD3). Based on heterologous expression in Saccharomyces cerevisiae, it was found that Sk-FAD2 converted C16-20 monounsaturated fatty acids to diunsaturated fatty acids by the introduction of a second double bond at the nu+3 position, while Sk-FAD3 recognized the omega3 position of C18 and C20. Furthermore, fatty acid analysis of major phospholipids suggested that Sk-FAD2 and Sk-FAD3 have no strong substrate specificity toward the lipid polar head group or the sn-positions of fatty acyl groups in phospholipids.

A multispecies-based taxonomic microarray reveals interspecies hybridization and introgression inSaccharomyces cerevisiae

A multispecies-based taxonomic microarray targeting coding sequences of diverged orthologous genes in Saccharomyces cerevisiae, Saccharomyces paradoxus, Saccharomyces mikatae, Saccharomyces bayanus, Saccharomyces kudriavzevii, Naumovia castellii, Lachancea kluyveri and Candida glabrata was designed to allow identification of isolates of these species and their interspecies hybrids. Analysis of isolates of several Saccharomyces species and interspecies hybrids demonstrated the ability of the microarray to differentiate these yeasts on the basis of their specific hybridization patterns. Subsequent analysis of 183 supposed S. cerevisiae isolates of various ecological and geographical backgrounds revealed one misclassified S. bayanus or Saccharomyces uvarum isolate and four aneuploid interspecies hybrids, one between S. cerevisiae and S. bayanus and three between S. cerevisiae and S. kudriavzevii. Furthermore, this microarray design allowed the detection of multiple introgressed S. paradoxus DNA fragments in the genomes of three different S. cerevisiae isolates. These results show the power of multispecies-based microarrays as taxonomic tools for the identification of species and interspecies hybrids, and their ability to provide a more detailed characterization of interspecies hybrids and recombinants.

Saccharomyces kluyverias a model organism to study pyrimidine degradation

The yeast Saccharomyces kluyveri (Lachancea kluyveri), a far relative of Saccharomyces cerevisiae, is not a widely studied organism in the laboratory. However, significant contributions to the understanding of nucleic acid precursors degradation in eukaryotes have been made using this model organism. Here we review eukaryotic pyrimidine degradation with emphasis on the contributions made with S. kluyveri and how this increases our understanding of human disease. Additionally, we discuss the possibilities and limitations of this nonconventional yeast as a laboratory organism.

Degradation of Pyrimidines in Saccharomyces Kluyveri: Transamination of β-Alanine

Beta-alanine is an intermediate in the reductive degradation of uracil. Recently we have identified and characterized the Saccharomyces kluyveri PYD4 gene and the corresponding enzyme β -alanine aminotransferase ( Sk Pyd4p), highly homologous to eukaryotic γ-aminobutyrate aminotransferase (GABA-AT). S. kluyveri has two aminotransferases, GABA aminotransferase ( Sk Uga1p) with 80% and Sk Pyd4p with 55% identity to S. cerevisiae GABA-AT. Sk Pyd4p is a typical pyridoxal phosphate-dependent aminotransferase, specific for α -ketoglutarate (α KG), β -alanine (BAL) and γ -aminobutyrate (GABA), showing a ping-pong kinetic mechanism involving two half-reactions and substrate inhibition. Sk Uga1p accepts only α KG and GABA but not BAL, thus only Sk Pydy4p belongs to the uracil degradative pathway.

A Second Pathway to Degrade Pyrimidine Nucleic Acid Precursors in Eukaryotes

Pyrimidine bases are the central precursors for RNA and DNA, and their intracellular pools are determined by de novo, salvage and catabolic pathways. In eukaryotes, degradation of uracil has been believed to proceed only via the reduction to dihydrouracil. Using a yeast model, Saccharomyces kluyveri, we show that during degradation, uracil is not reduced to dihydrouracil. Six loci, named URC1–6 (for uracil catabolism), are involved in the novel catabolic pathway. Four of them, URC3,5, URC6, and URC2 encode urea amidolyase, uracil phosphoribosyltransferase, and a putative transcription factor, respectively. The gene products of URC1 and URC4 are highly conserved proteins with so far unknown functions and they are present in a variety of prokaryotes and fungi. In bacteria and in some fungi, URC1 and URC4 are linked on the genome together with the gene for uracil phosphoribosyltransferase ( URC6). Urc1p and Urc4p are therefore likely the core components of this novel biochemical pathway. A combination of genetic and analytical chemistry methods demonstrates that uridine monophosphate and urea are intermediates, and 3-hydroxypropionic acid, ammonia and carbon dioxide the final products of degradation. The URC pathway does not require the presence of an active respiratory chain and is therefore different from the oxidative and rut pathways described in prokaryotes, although the latter also gives 3-hydroxypropionic acid as the end product. The genes of the URC pathway are not homologous to any of the eukaryotic or prokaryotic genes involved in pyrimidine degradation described to date.