Formation Of Viable Spores Research Articles

The Smk1 MAPK and Its Activator, Ssp2, Are Required for Late Prospore Membrane Development in Sporulating Saccharomyces cerevisiae.

During sporulation in the budding yeast Saccharomyces cerevisiae, proper development of the prospore membrane is necessary for the formation of viable spores. The prospore membrane will eventually become the plasma membrane of the newly formed haploid spore and also serves as the template for the deposition of the spore wall. The prospore membrane is generated de novo during meiosis II and the growing edge of the prospore membrane is associated with the Leading Edge Protein (LEP) complex. We find that the Smk1 MAP kinase, along with its activator Ssp2, transiently localizes with the LEP during late meiosis II. SSP2 is required for the leading edge localization of Smk1; this localization is independent of the activation state of Smk1. Like other LEP components, the localization of Smk1 at the leading edge also depends on Ady3. Although prospore membrane development begins normally in smk1 and ssp2 mutants, late prospore membrane formation is disrupted, with the formation of ectopic membrane compartments. Thus, MAP kinase signaling plays an important role in the formation of the prospore membrane.

The unique DKxanthene secondary metabolite family from the myxobacterium Myxococcus xanthus is required for developmental sporulation

Under starvation conditions myxobacteria form multicellular fruiting bodies in which vegetative cells differentiate into heat- and desiccation-resistant myxospores. Myxobacteria in general are a rich source of secondary metabolites that often exhibit biological activities rarely found in nature. Although the involvement of a yellow compound in sporulation and fruiting body formation of Myxococcus xanthus was described almost 30 years ago, the chemical principle of the pigment remained elusive. This work presents the isolation and structure elucidation of a unique class of pigments that were named DKxanthenes (DKX). The corresponding biosynthetic gene cluster was identified, and DKX-negative mutants were constructed to investigate the physiological role of DKX during development. In these mutants, fruiting body formation was delayed. Moreover, severely reduced amounts of viable spores were observed after 120 h of starvation, whereas no viable spores were formed at all after 72 h. The addition of purified DKX to the mutants resulted in the formation of viable spores after 72 h. Even though an antioxidative activity could be assigned to DKX, the true biochemical mechanism underlying the complementation remains to be elucidated.

Meiotic segregation, synapsis, and recombination checkpoint functions require physical interaction between the chromosomal proteins Red1p and Hop1p.

In yeast, HOP1 and RED1 are required during meiosis for proper chromosome segregation and the consequent formation of viable spores. Mutations in either HOP1 or RED1 create unique as well as overlapping phenotypes, indicating that the two proteins act alone as well as in concert with each other. To understand which meiotic processes specifically require Red1p-Hop1p hetero-oligomers, a novel genetic screen was used to identify a single-point mutation of RED1, red1-K348E, that separates Hop1p binding from Red1p homo-oligomerization. The Red1-K348E protein is stable, phosphorylated in a manner equivalent to Red1p, and undergoes efficient homo-oligomerization; however, its ability to interact with Hop1p both by two-hybrid and coimmunoprecipitation assays is greatly reduced. Overexpression of HOP1 specifically suppresses red1-K348E, supporting the idea that the only defect in the protein is a reduced affinity for Hop1p. red1-K348E mutants exhibit reduced levels of crossing over and spore viability and fail to undergo chromosome synapsis, thereby implicating a role for Red1p-Hop1p hetero-oligomers in these processes. Furthermore, red1-K348E suppresses the sae2/com1 defects in meiotic progression and sporulation, indicating a previously unknown role for HOP1 in the meiotic recombination checkpoint.

High copy number suppression of the meiotic arrest caused by a dmc1 mutation: REC114 imposes an early recombination block and RAD54 promotes a DMC1-independent DSB repair pathway.

DMC1, the meiosis-specific eukaryotic homologue of bacterial recA, is required for completion of meiotic recombination and cell cycle progression past prophase. In a dmc1 mutant, double strand break recombination intermediates accumulate and cells arrest in prophase. We isolated genes which, when present at high copy numbers, suppress the meiotic arrest phenotype conferred by dmc1 mutations. Among the genes isolated were two which suppress arrest by altering the recombination process. REC114 suppresses formation of double strand break (DSB) recombination intermediates. The low viability of spores produced by dmc1 mutants carrying high copy numbers of REC114 is rescued when reductional segregation is bypassed by mutation of spo13. High copy numbers of RAD54 suppress dmc1 arrest, promote DSB repair, and allow formation of viable spores following reductional segregation. Analysis of the combined effects of a null mutation in RED1, a gene required for meiotic chromosome structure, with null mutations in RAD54 and DMC1 shows that RAD54, while not normally important for repair of DSBs during meiosis, is required for efficient repair of breaks by the intersister recombination pathway that operates in red1 dmc1 double mutants. Over-expression of REC114 suppresses meiotic arrest by preventing formation of DSBs. High copy numbers of RAD54 activate a DMC1-independent mechanism that promotes repair of DSBs by homology-mediated recombination. The ability of RAD54 to promote DMC1-independent recombination is proposed to involve suppression of a constraint that normally promotes recombination between homologous chromatids rather than sisters.

Functions of the yeast meiotic recombination genes, MRE11 and MRE2

Mutants defective in meiotic recombination were isolated using a disomic haploid strain of S. cerevisiae, and were classified into 11 genes. Two, MRE2 and MRE11, are new genes and nine are previously identified genes. The mre2 and mrell deletion mutants are proficient in mitotic recombination, but are defective in meiotic recombination and in formation of viable spores. The spore inviability, however, is alleviated by an additional mutation, spo13, which bypasses meiosis I. In addition, neither meiosis specific DSBs at recombination hot-spots nor formation of synaptonemal complex occur in either mutant. Therefore, these two genes are involved in the formation of DSBs in meiotic recombination. While a temperature sensitive mre11-1 mutant is able to form DSBs at a permissive temperature, the formed DSBs are unable to resect at non permissive temperature. Therefore, the MRE11 gene is also involved in some step of the repair process after the DSB formation. Analysis of properties of the mre11 disruption mutant as well as the xrs2 mutant showed a similarity to those of the rad50 disruptant. We found that the mre11 disruption mutation is epistatic to rad50S mutation, as the xrs2 deletion mutation is epistatic to rad5OS with regard to DSBs. Therefore, these three genes form an epistatic group. Interaction of the Mre11 protein with the Rad50 and the Xrs2 protein as well as alone was shown in vivo using the two-hybrid system. The MRE2 gene encodes a protein containing two sets of RRM. Deficiency of recombination in a mre2 mutant that has an amino acid substitution in the N-terminal RRM can be suppressed by the MER2 gene on the multicopy plasmid. Further analysis showed that the Mre2 protein is involved in meiosis-specific splicing of the MER2 transcripts in cooperation with the Mer1 protein. In conclusion, MRE genes are involved in the initiation of meiotic recombination through the formation of DSBs at recombination hot-spots in S. cerevisiae.

Meiosis in Vitro: Sporogenesis in Cultured Fern Plants

Sporangia containing mature, functional spores can be induced to form on pinnae of Todea barbara sporophytes maintained in nutrient culture. Meiosis is initiated in the sporogenous cells and proceeds to completion in vitro with the formation of viable spores. Sporangial development, meiosis and the production of spores occur in plants which exhibit signs of senescence. Fertile leaves are characterized by a reduction in chlorophyll concentration and total cellular nitrogen when compared to sterile leaves. The data suggest that the process of sporogenesis may be correlated with a reduction in metabolic activity of the leaf.

MEIOSIS IN VITRO: SPOROGENESIS IN CULTURED FERN PLANTS

Sporangia containing mature, functional spores can be induced to form on pinnae of Todea barbara sporophytes maintained in nutrient culture. Meiosis is initiated in the sporogenous cells and proceeds to completion in vitro with the formation of viable spores. Sporangial development, meiosis and the production of spores occur in plants which exhibit signs of senescence. Fertile leaves are characterized by a reduction in chlorophyll concentration and total cellular nitrogen when compared to sterile leaves. The data suggest that the process of sporogenesis may be correlated with a reduction in metabolic activity of the leaf.