Depletion of H2A-H2B dimers in Saccharomyces cerevisiae triggers meiotic arrest by reducing IME1 expression and activating the BUB2-dependent branch of the spindle checkpoint.

In the yeast Saccharomyces cerevisiae, diploid strains carrying homozygous hta1-htb1Delta mutations express histone H2A-H2B dimers at a lower level than do wild-type cells. Although this mutation has only minor effects on mitotic growth, it causes an arrest in sporulation prior to the first meiotic division. In this report, we show that the hta1-htb1Delta mutant exhibits reduced expression of early and middle-sporulation-specific genes and that the meiotic arrest of the hta1-htb1Delta mutant can be partially bypassed by overexpression of IME1. Additionally, deletions of BUB2 or BFA1, components of one branch of the spindle checkpoint pathway, bypass the meiotic arrest. Mutations in the other branch of the pathway or in the pachytene checkpoint are unable to suppress the meiotic block. These observations indicate that depletion of the H2A-H2B dimer blocks sporulation by at least two mechanisms: disruption of the expression of meiotic regulatory genes and activation of the spindle checkpoint. Our results show that the failure to progress through the meiotic pathway is not the result of global chromosomal alterations but that specific aspects of meiosis are sensitive to depletion of the H2A-H2B dimer.

Analysis of the meiotic role of the mitochondrial ribosomal proteins Mrps17 and Mrpl37 in Saccharomyces cerevisiae.

Sporulation in the yeast Saccharomyces cerevisiae is a complex and tightly regulated pathway that involves the induction of a large number of genes. We have identified MRPS17 in a cDNA library enriched for sporulation-specific genes. Homology searches show that the first one-third of Mrps17 has strong sequence similarity to bacterial S17 proteins, suggesting that Mrps17 is a potential mitochondrial ribosomal protein. This is further supported by the fact that mrps17Delta cells are respiratory-deficient and that a Mrps17-GFP fusion localizes to the mitochondria. We have confirmed by Northern blot analysis that both MRPS17 and MRPL37 are strongly induced during the middle stages of sporulation and that this induction is dependent on the presence of a middle sporulation element (MSE) in the promoters of these genes. Interestingly, we found that Mrps17 and Mrpl37, but not other mitochondrial ribosomal proteins, accumulate during the middle stages of sporulation. These results suggest that Mrps17 and Mrpl37 may have additional meiosis-specific roles.