Abstract
BackgroundThymidylate synthase is a housekeeping gene, designated ancient due to its role in DNA synthesis and ubiquitous phyletic distribution. The genomic sequences were characterized coding for thymidylate synthase in two species of the genus Trichinella, an encapsulating T. spiralis and a non-encapsulating T. pseudospiralis.MethodsBased on the sequence of parasitic nematode Trichinella spiralis thymidylate synthase cDNA, PCR techniques were employed.ResultsEach of the respective gene structures encompassed 6 exons and 5 introns located in conserved sites. Comparison with the corresponding gene structures of other eukaryotic species revealed lack of common introns that would be shared among selected fungi, nematodes, mammals and plants. The two deduced amino acid sequences were 96% identical. In addition to the thymidylate synthase gene, the intron-less retrocopy, i.e. a processed pseudogene, with sequence identical to the T. spiralis gene coding region, was found to be present within the T. pseudospiralis genome. This pseudogene, instead of the gene, was confirmed by RT-PCR to be expressed in the parasite muscle larvae.ConclusionsIntron load, as well as distribution of exon and intron phases in thymidylate synthase genes from various sources, point against the theory of gene assembly by the primordial exon shuffling and support the theory of evolutionary late intron insertion into spliceosomal genes. Thymidylate synthase pseudogene expressed in T. pseudospiralis muscle larvae is designated a retrogene.
Highlights
Thymidylate synthase is a housekeeping gene, designated ancient due to its role in DNA synthesis and ubiquitous phyletic distribution
Of note is that high thymidylate synthase activity found in T. spiralis and T. pseudospiralis muscle larvae does not appear to vary in connection with the difference in the intracellular niche occupied by two species
T. spiralis and T. pseudospiralis thymidylate synthase genes share the same structure and show 11 substitutions at the deduced amino acid sequence level T. spiralis and T. pseudospiralis thymidylate synthase genes consist of 6 exons, intervened by 5 introns, marked by GT/AG donor/acceptor splice sites (Figure 1)
Summary
Thymidylate synthase is a housekeeping gene, designated ancient due to its role in DNA synthesis and ubiquitous phyletic distribution. The genomic sequences were characterized coding for thymidylate synthase in two species of the genus Trichinella, an encapsulating T. spiralis and a non-encapsulating T. pseudospiralis. Our previous studies documented high thymidylate synthase activity persisting in developmentally arrested muscle larvae of both species, as well as in T. spiralis adult forms [4,5]. High enzyme activity is known to accompany proliferation, as well as to persist in certain growtharrested systems where it is not associated with cell division cf [5]. Of note is that high thymidylate synthase activity found in T. spiralis and T. pseudospiralis muscle larvae does not appear to vary in connection with the difference in the intracellular niche occupied by two species
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