Histone proteins play structural and functional roles in all nuclear processes. They undergo different types of covalent modifications, defined in their ensemble as epigenetic because changes in DNA sequences are not involved. Histone acetylation emerges as a central switch that allows interconversion between permissive and repressive chromatin domains in terms of transcriptional competence. The mechanisms underlying the histone acetylation-dependent control of gene expression include a direct effect on the stability of nucleosomal arrays and the creation of docking sites for the binding of regulatory proteins. Histone acetyltransferases and deacetylases are, respectively, the enzymes devoted to the addition and removal of acetyl groups from lysine residues on the histone N-terminal tails. The enzymes exert fundamental roles in developmental processes and their deregulation has been linked to the progression of diverse human disorders, including cancer.Key words: gene expression, transcription, HATs, HDACs, nucleosome.

Seven Sm proteins associate with U1, U2, U4 and U5 spliceosomal snRNAs and influence snRNP biogenesis. Here we describe a novel set of Sm‐like (Lsm) proteins in Saccharomyces cerevisiae that interact with each other and with U6 snRNA. Seven Lsm proteins co‐immunoprecipitate with the previously characterized Lsm4p (Uss1p) and interact with each other in two‐hybrid analyses. Free U6 and U4/U6 duplexed RNAs co‐immunoprecipitate with seven of the Lsm proteins that are essential for the stable accumulation of U6 snRNA. Analyses of U4/U6 di‐snRNPs and U4/U6·U5 tri‐snRNPs in Lsm‐depleted strains suggest that Lsm proteins may play a role in facilitating conformational rearrangements of the U6 snRNP in the association–dissociation cycle of spliceosome complexes. Thus, Lsm proteins form a complex that differs from the canonical Sm complex in its RNA association(s) and function. We discuss the possible existence and functions of alternative Lsm complexes, including the likelihood that they are involved in processes other than pre‐mRNA splicing.

Genetic information is packaged in the highly dynamic nucleoprotein structure called chromatin. Many biological processes are regulated via post-translational modifications of key proteins. Acetylation of lysine residues at the N-terminal histone tails is one of the most studied covalent modifications influencing gene regulation in eukaryotic cells. This review focuses on the role of enzymes involved in controlling both histone and non-histone proteins acetylation levels in the cell, with particular emphasis on their effects on cancer.

Through a combination of in vitro snRNP reconstitution, photocross-linking and immunoprecipitation techniques, we have investigated the interaction of proteins with the spliceosomal U6 snRNA in U6 snRNPs, U4/U6 di-snRNPs and U4/U6.U5 tri-snRNPs. Of the seven Lsm (Sm-like) proteins that associate specifically with this spliceosomal snRNA, three were shown to contact the RNA directly, and to maintain contact as the U6 RNA is incorporated into tri-snRNPs. In tri-snRNPs, the U5 snRNP protein Prp8 contacts position 54 of U6, which is in the conserved region that contributes to the formation of the catalytic core of the spliceosome. Other tri-snRNP-specific contacts were also detected, indicating the dynamic nature of protein interactions with this important snRNA. The uridine-rich extreme 3′ end of U6 RNA was shown to be essential but not sufficient for the association of the Lsm proteins. Interestingly, the Lsm proteins associate efficiently with the 3′ half of U6, which contains the 3′ stem-loop and uridine-rich 3′ end, suggesting that the Lsm and Sm proteins may recognize similar features in RNAs.

We have analyzed at both low and high resolution the distribution of nucleosomes over the Saccharomyces cerevisiae ADH2 promoter region in its chromosomal location, both under repressing (high-glucose) conditions and during derepression. Enzymatic treatments (micrococcal nuclease and restriction endonucleases) were used to probe the in vivo chromatin structure during ADH2 gene activation. Under glucose-repressed conditions, the ADH2 promoter was bound by a precise array of nucleosomes, the principal ones positioned at the RNA initiation sites (nucleosome +1), at the TATA box (nucleosome -1), and upstream of the ADR1-binding site (UAS1) (nucleosome -2). The UAS1 sequence and the adjacent UAS2 sequence constituted a nucleosome-free region. Nucleosomes -1 and +1 were destabilized soon after depletion of glucose and had become so before the appearance of ADH2 mRNA. When the transcription rate was high, nucleosomes -2 and +2 also underwent rearrangement. When spheroplasts were prepared from cells grown in minimal medium, detection of this chromatin remodeling required the addition of a small amount of glucose. Cells lacking the ADR1 protein did not display any of these chromatin modifications upon glucose depletion. Since the UAS1 sequence to which Adr1p binds is located immediately upstream of nucleosome -1, Adr1p is presumably required for destabilization of this nucleosome and for aiding the TATA-box accessibility to the transcription machinery.

The LSM4 gene of Saccharomyces cerevisiae codes for an essential protein involved in pre-mRNA splicing and also in mRNA decapping, a crucial step for mRNA degradation. We previously demonstrated that the first 72 amino acids of the Kluyveromyces lactis Lsm4p (KlLsm4p), which contain the Sm-like domains, can restore cell viability in bothK. lactis and S. cerevisiae cells not expressing the endogenous protein. However, the absence of the carboxy-terminal region resulted in a remarkable loss of viability in stationary phase cells (Mazzoni and Falcone, 2001). Herein, we demonstrate that S. cerevisiae cells expressing the truncated LSM4 protein of K. lactisshowed the phenotypic markers of yeast apoptosis such as chromatin condensation, DNA fragmentation, and accumulation of reactive oxygen species. The study of deletion mutants revealed that apoptotic markers were clearly evident also in strains lacking genes involved in mRNA decapping, such as LSM1, DCP1, andDCP2, whereas a slight effect was observed in strains lacking the genes DHH1 and PAT1. This is the first time that a connection between mRNA stability and apoptosis is reported in yeast, pointing to mRNA decapping as the crucial step responsible of the observed apoptotic phenotypes.