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SSn6-Tup1 seems to actively recruit deactylase activities to deacetylate adjacent nucleosomes and to promote tup1-histone interactions. Histone hyperacetylation brought on by combined mutations in genes that encode certain histone deacetylases (HDACs) does away with Ssn6-Tup1 repression.

The yeast transcriptional adaptor, Gcn5p selectively acetylates histones H3 and H4 at specific lysines in the amino-terminal domains. Acetylation occurs at the lysine 14 of H3 and at lysines 8 and 16 of H4. Newly formed yeast H3 prefers acetylation at lysine 9. Gcn5p also acetylates lysines 5 and 12 in H4 during chromatin assembly of several organisms. The set of lysines acetylated by Gcn5p do not overlap with the sites generally used by the type B histone acetyltransferases for histone deposition and chromatin assembly.

A muscle specific transcription factor that can induce myogenesis in a variety of cell types is myogenin. To test myogenin's role in embryonic muscle development, homozygous mice were generated for a targeted mutation in the myogenin gene. Though these mice survived fetal development, they were born immobile and died immediately after birth. They have severe reduction in their skeletal muscle throughout their body. Myogenin is thus an essential component for the skeletal muscle formation during embryogenesis. Mice lacking myogenin have gross rib defects, which are not revealed at the cellular level as bone and cartilage appear histologically normal.

Antigenic variation plays a vital role in the pathogenesis of many infectious bacteria and protozoa including Borrelia burgdorferi, the causative agent of Lyme disease. VlsE, a 35 kDa surface-exposed lipoprotein, undergoes antigenic variation during B. burgdorferi infection of mammalian hosts, and is believed to be a critical mechanism by which the spirochetes evade immune clearance. Random, segmental recombination between the expressed vlsE gene and adjacent vls silent cassettes generates a large number of different VlsE variants within the infected host. Although the occurrence and importance of vlsE sequence variation is well established, little is known about the biological mechanism of vlsE recombination. To identify factors important in antigenic variation and vlsE recombination, we screened transposon mutants of genes known to be involved in DNA recombination and repair for their effects on infectivity and vlsE recombination. Several mutants, including those in BB0023 (ruvA), BB0022 (ruvB), BB0797 (mutS), and BB0098 (mutS-II), showed reduced infectivity in immunocompetent C3H/HeN mice. Mutants in ruvA and ruvB exhibited greatly reduced rates of vlsE recombination in C3H/HeN mice, as determined by restriction fragment polymorphism (RFLP) screening and DNA sequence analysis. In severe combined immunodeficiency (C3H/scid) mice, the ruvA mutant retained full infectivity; however, all recovered clones retained the 'parental' vlsE sequence, consistent with low rates of vlsE recombination. These results suggest that the reduced infectivity of ruvA and ruvB mutants is the result of ineffective vlsE recombination and underscores the important role that vlsE recombination plays in immune evasion. Based on functional studies in other organisms, the RuvAB complex of B. burgdorferi may promote branch migration of Holliday junctions during vlsE recombination. Our findings are consistent with those in the accompanying article by Dresser et al., and together these studies provide the first examples of trans-acting factors involved in vlsE recombination. [ABSTRACT FROM AUTHOR]

The Tup1-Ssn6 corepressor complex in Saccharomyces cerevisiae represses the transcription of a diverse set of genes. Chromatin is an important component of Tup1Ssn6-mediated repression. Tup1 binds to underacetylated histone tails and requires multiple histone deacetylases (HDACs) for its repressive functions. Here, we describe physical interactions of the corepressor complex with the class I HDACs Rpd3, Hos2, and Hos1. In contrast, no in vivo interaction was observed between Tup-Ssn6 and Hda1, a class II HDAC. We demonstrate that Rpd3 interacts with both Tup1 and Ssn6. Rpd3 and Hos2 interact with Ssn6 independently of Tup1 via distinct tetratricopeptide domains within Ssn6, suggesting that these two HDACs may contact the corepressor at the same time. [ABSTRACT FROM AUTHOR]

The Gcn5p histone acetyltransferase exhibits a limited substrate specificity in vitro. However, neither the specificity of this enzyme in vivo nor the importance of particular acetylated residues to transcription or cell growth are well defined. To probe these questions, we mutated specific lysines in the N-termini of histones H3 and H4 and examined the effects of these mutations in yeast strains with and without functional GCN5. We found that in vivo, GCN5 is required either directly or indirectly for the acetylation of several sites in H3 and H4 in addition to those recognized by the recombinant enzyme in vitro. Moreover, in the absence of GCN5, cells accumulate in G2/M indicating that Gcn5p functions are important for normal cell-cycle progression. Mutation of K14 in H3, which serves as the major target of recombinant Gcn5p acetylation in vitro, confers a strong, synthetic growth defect in gcn5 cells. Synergistic growth defects were also observed in gcn5 cells carrying mutations in lysine pairs (K8/K16 or K5/K12) in histone H4. Strikingly, simultaneous mutation of K14 in H3 and K8 and K16 in H4 to arginine, or deletion of either the H3 or the H4 N-terminal tail, results in the death of gcn5 cells. Mutation of these same three sites to glutamine is not lethal. Indeed, this combination of mutations largely bypasses the need for GCN5 for transcriptional activation by Gal4—VP16, supporting an important role for histone acetylation in Gcn5pmediated regulation of transcription. Our data indicate that acetylation of particular lysines in histones H3 and H4 serves both unique and overlapping functions important for normal cell growth, and that a critical overall level of histone acetylation is essential for cell viability. [ABSTRACT FROM AUTHOR]