We previously showed that Arabidopsis thaliana histone acetyltransferase TAF1/HAF2 is required for the light regulation of growth and gene expression, and we show here that histone acetyltransferase GCN5 and histone deacetylase HD1/HDA19 are also involved in such regulation. Mutation of GCN5 resulted in a long-hypocotyl phenotype and reduced light-inducible gene expression, whereas mutation of HD1 induced opposite effects. The double mutant gcn5 hd1 restored a normal photomorphogenic phenotype. By contrast, the double mutant gcn5 taf1 resulted in further loss of light-regulated gene expression. gcn5 reduced acetylation of histones H3 and H4, mostly on the core promoter regions, whereas hd1 increased acetylation on both core and more upstream promoter regions. GCN5 and TAF1 were both required for H3K9, H3K27, and H4K12 acetylation on the target promoters, but H3K14 acetylation was dependent only on GCN5. Interestingly, gcn5 taf1 had a cumulative effect mainly on H3K9 acetylation. On the other hand, hd1 induced increased acetylation on H3K9, H3K27, H4K5, and H4K8. GCN5 was also shown to be directly associated with the light-responsive promoters. These results suggest that acetylation of specific histone Lys residues, regulated by GCN5, TAF1, and HD1, is required for light-regulated gene expression.

Among the most numerous objects in the biosphere, phages show enormous diversity in morphology and genetic content. We have sequenced 7 T4-like phages and compared their genome architecture. All seven phages share a core genome with T4 that is interrupted by several hyperplastic regions (HPRs) where most of their divergence occurs. The core primarily includes homologues of essential T4 genes, such as the virion structure and DNA replication genes. In contrast, the HPRs contain mostly novel genes of unknown function and origin. A few of the HPR genes that can be assigned putative functions, such as a series of novel Internal Proteins, are implicated in phage adaptation to the host. Thus, the T4-like genome appears to be partitioned into discrete segments that fulfil different functions and behave differently in evolution. Such partitioning may be critical for these large and complex phages to maintain their flexibility, while simultaneously allowing them to conserve their highly successful virion design and mode of replication.

Histone acetyltransferases, which are able to acetylate histone and non-histone proteins, play important roles in gene regulation. Many histone acetyltransferases are related to yeast Gcn5, a component of two transcription regulatory complexes SAGA and ADA. In this work, by characterizing a mutation in the Arabidopsis GCN5 gene (AtGCN5) we studied the regulatory function of this gene in controlling floral meristem activity. We show that in addition to pleiotropic effects on plant development, this mutation also leads to the production of terminal flowers. The flowers show homeotic transformations of petals into stamens and sepals into filamentous structures and produce ectopic carpels. The phenotypes correlate to an expansion of the expression domains within floral meristems of the key regulatory genes WUSCHEL (WUS) and AGAMOUS (AG). These results suggest that AtGCN5 is required to regulate the floral meristem activity through the WUS/AG pathway. This study brings new elements on the elucidation of specific developmental pathways regulated by AtGCN5 and on the control mechanism of meristem regulatory gene expression.