MellorLab

Walter, W.,  D. Clynes, et al. (2008).
"14-3-3 interaction with histone H3 involves dual modification pattern of phosphoacetylation."
Mol Cell Biol.

Histone modifications occur in precise patterns and are proposed to signal the recruitment of effector molecules that profoundly impact chromatin structure, gene regulation, and cell cycle events. The linked modifications serine 10 phosphorylation and lysine 14 acetylation on histone H3 (H3S10phK14ac), modifications conserved from yeast to humans, are crucial for transcriptional activation of many genes. However, the mechanism of H3S10phK14ac involvement in these processes is unclear. To shed light on the role of this dual modification, we utilized H3 peptide affinity assays to identify H3S10phK14ac-interacting proteins. We found that the interaction of the known phospho-binding 14-3-3 proteins with H3 is dependent on the presence of both of these marks, not just phosphorylation alone. This is true of mammalian 14-3-3 proteins as well as the yeast homologues Bmh1 and Bmh2. The importance of acetylation in this interaction is also seen in vivo, where K14 acetylation is required for optimal Bmh1 recruitment to the GAL1 promoter during transcriptional activation.

Sherriff,J.A., N.A.Kent, et al. (2007).

 "The Isw2 chromatin-remodeling ATPase cooperates with the Fkh2 transcription factor to repress transcription of the B-type cyclin gene CLB2." Mol Cell Biol 27(8): 2848-60.

                Forkhead (Fkh) transcription factors influence cell death, proliferation, and differentiation and the cell cycle. In Saccharomyces cerevisiae, Fkh2 both activates and represses transcription of CLB2, encoding a B-type cyclin. CLB2 is expressed during G(2)/M phase and repressed during G(1). Fkh2 recruits the coactivator Ndd1, an interaction which is promoted by Clb2/Cdk1-dependent phosphorylation of Ndd1, suggesting that CLB2 is autoregulated. Ndd1 is proposed to function by antagonizing Fkh2-mediated repression, but nothing is known about the mechanism. Here we ask how Fkh2 represses CLB2. We show that Fkh2 controls a repressive chromatin structure that initiates in the early coding region of CLB2 and spreads up the promoter during the M and G(1) phases. The Isw2 chromatin-remodeling ATPase cooperates with Fkh2 to remodel the chromatin and repress CLB2 expression throughout the cell cycle. In addition, the related factors Isw1 and Fkh1 configure the chromatin at the early coding region and negatively regulate CLB2 expression but only during G(2)/M phase. Thus, the cooperative actions of two forkhead transcription factors and two chromatin-remodeling ATPases combine to regulate CLB2. We propose that chromatin-mediated repression by Isw1 and Isw2 may serve to limit activation of CLB2 expression by the Clb2/Cdk1 kinase during G(2)/M and to fully repress expression during G(1).

Mellor, J. (2006).
"It takes a PHD to read the histone code."
Cell 126(1): 22-4.

 The pattern of histone modifications, called the histone code, influences transitions between chromatin states and the regulation of transcriptional activity. Four recent papers describe how plant homeodomain (PHD) finger proteins read part of this code. The PHD finger may promote both gene expression and repression through interactions with trimethylated lysine 4 on histone 3 (H3K4), a universal modification at the beginning of active genes.

Mellor, J. (2006).
"Imitation switch complexes."
Ernst Schering Res Found Workshop(57): 61-87.

                The imitation switch (ISWI) family of chromatin remodelling ATPases is found in organisms ranging from yeast to mammals. ISWI ATPases assemble chromatin and slide and space nucleosomes, making the chromatin template fluid and allowing appropriate regulation of events such as transcription, DNA replication, recombination and repair. The site of action of the ATPases is determined, in part by the tissue type in which the enzyme is expressed and in part by the nature of the proteins associated with the enzyme. The ISWI complexes are generally conserved in composition and function across species. Roles in gene expression and DNA replication in heterochromatin, gene activation and repression in euchromatin, and functions related to maintaining chromosome architecture are associated with different complexes. Defects in ISWI-associated proteins may be associated with neurodegenerative disease, anencephaly, William's syndrome and melanotic tumours. Finally, the mechanism by which yeast Isw Ib influences gene transcription is discussed.

Mellor, J. (2006).
"Dynamic nucleosomes and gene transcription."
Trends Genet 22(6): 320-9.

                Gene transcription occurs on a nucleosomal template known as chromatin. The recruitment of the transcriptional regulators and the transcription machinery to promoter chromatin is coordinated by a genetic code on the DNA and an epigenetic code on the histone proteins. Chromatin is not a stable environment; rather, the histones, the transcription regulatory proteins and the enzymes that modify and mobilise nucleosomes are in a dynamic state. Thus, at any one time, the factors associated with a region will cooperate or compete to change the rate of inter-conversion between permissive and non-permissive chromatin states, leading to activation or repression of transcription. Here, new concepts such as dynamic nucleosomes and a dynamic histone code in gene transcription are explored.

Mellor, J. (2005).
"The dynamics of chromatin remodeling at promoters."
Mol Cell 19(2): 147-57.

               The nucleosome, the structural unit of chromatin, is known to play a central role in regulating gene transcription from promoters. The last seven years have spawned a vast amount of data on the enzymes that remodel and modify nucleosomes and the rules governing how transcription factors interact with the epigenetic code on histones. Yet despite this effort, there has yet to emerge a unifying mechanism by which nucleosomes are remodeled during gene regulation. Recent advances have allowed nucleosome dynamics on promoters to be studied in real time, dramatically changing how we think about gene regulation on chromatin templates.

Mellor, J. and A. Morillon (2004).
"ISWI complexes in Saccharomyces cerevisiae."  
Biochim Biophys Acta 1677(1-3): 100-12.

                The imitation switch (ISWI) class of chromatin remodeling ATPase is ubiquitous in eukaryotes. It is becoming clear that these enzymes exist as part of larger complexes and the nature of the associated proteins dictate the function associated with a complex both in biochemical assays and in the cell. Much progress has been made in understanding these relationships in the budding yeast Saccharomyces cerevisiae, containing two ATPases, Isw1p and Isw2p. This has been aided by the ease of genetic manipulation, by a number of systematic screens designed to specifically detect ISWI function and by the plethora of data generated from a number of global screens for function. At present, many functions for yeast Isw1p and Isw2p are related to effects on RNA levels and are associated with the controlled repression of gene expression that crudely fall into three types: displacement of the basal transcription machinery to repress or silence transcription of genes (Isw2 complex and Isw1/Ioc3 complex); control of the activation of expression leading to coordination of transcription elongation; and efficient termination of transcription (Isw1/Ioc4/Ioc2 complex). The latter two functions are regulated by specific phosphorylation of residues within the carboxy terminal domain (CTD) of the largest subunit of RNA polymerase II (RNAPII). Other functions may relate to the ability of ISWI complex to displace transcription factors or enzymes from the template. Other ISWI-containing complexes that have yet to be characterized indicate that much remains to be learnt about yeast ISWI itself and importantly, how the various forms cooperate with different classes of chromatin remodeling ATPase, complexes containing histone acetylases, deacetylases, methylases and both DNA and RNA polymerases.