We have isolated Saccharomyces cerevisiae mutants bearing deletions of one or the other of the two divergently transcribed gene pairs encoding H2A and H2B. The deletions produced diverse effects on the yeast life cycle. Deletion of TRT1, one of the H2A-H2B gene pair sets, affected mitotic growth, sporulation, spore germination, the heat shock response, and exit from the stationary phase; deletion of TRT2, the other H2A-H2B gene pair set, had negligible effects on these same processes. Using a genetic complementation assay, we found that the differential effects of the deletions could be attributed to two features of the gene sets: first, the expression of the TRTI gene pair, but not the TRT2 gene pair, could compensate for the absence of its partner; second, the protein subtypes encoded by the two gene pairs appear to have different functions in the heat shock response.Most DNA in eucaryotic chromosomes is associated with four core histones in a structure known as the nucleosome (18). It has long been assumed, but until recently never directly demonstrated, that nucteosomes are essential for chromosomes to function properly. The discovery that Saccharomyces cerevisiae contains only two genes for each core histone has made it possible to assess the in vivo function of nucleosomes by genetic analysis (10). Three points have emerged from such studies. First, H2A and H2B are essential proteins in yeast, since strains bearing frameshift mutations in both copies of the genes encoding H2A (HTAI and HTA2) or the genes encoding H2B (HTBI and HTB2) are inviable (5,15,25). Second, H2B may be required for mitosis, since a strain bearing a conditionally expressed HTB2 gene arrests late in the cell cycle when the synthesis of H2B is turned off (6). Finally, the maintenance of normal H2A-H2B dimer stoichiometry is a prerequisite for proper chromosome segregation, since overexpression of the genes for H2A and H2B decreases the fidelity of mitotic chromosome transmission (19).The last work (19) also suggested how the cell may regulate the formation of nucleosomes. Chromosome loss was increased only when H2A and H2B were jointly overexpressed and not when either protein was overexpressed individually. This result implies that the H2A-H2B dimer, and not H2A or H2B alone, regulates nucleosome formation in vivo. A regulatory role for the H2A-H2B dimer may also be reflected in the genomic arrangement of the HTA and HTB genes in yeast. These genes are located in two divergently transcribed TRT loci (TRTJ and TRT2), each of which contains one HTA gene and one HTB gene (Fig. 1A and B) (10). We have shown that the HTA and HTB genes at each locus are expressed at the same time during the cell cycle and that the genes at TRT1 are regulated by the same promoter elements (9,11,21). This observation, coupled with the apparent regulatory role of the H2A-H2B dimer, suggests that it may be more accurate to consider the TRT loci rather than the individual HTA and HTB genes as the regulatory units for H2A and H2B production.
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