Linking Global Histone Acetylation to the Transcription Enhancement of X-chromosomal Genes in Drosophila Males

Abstract

It has become well established for several genes that targeting of histone acetylation to promoters is required for the activation of transcription. In contrast, global patterns of acetylation have not been ascribed to any particular regulatory function. In Drosophila, a specific modification of H4, acetylation at lysine 16, is enriched at hundreds of sites on the male X chromosome due to the activity of the male-specific lethal (MSL) dosage compensation complex. Utilizing chromatin immunoprecipitation, we have determined that H4Ac16 is present along the entire length of X-linked genes targeted by the MSL complex with relatively modest levels of acetylation at the promoter regions and high levels in the middle and/or 3 end of the transcription units. We propose that global acetylation by the MSL complex increases the expression of X-linked genes by facilitating transcription elongation rather than by enhancing promoter accessibility. We have also determined that H4Ac16 is absent from a region of the X chromosome that includes a gene known to be dosage-compensated by a MSL-independent mechanism. This study represents the first biochemical interpretation of the very large body of cytological observations on the chromosomal distribution of the MSL complex.Many post-translational modifications of the highly conserved histone N-terminal tails are linked to transcriptional regulation (1), with the most widely studied modification being acetylation of histones H3 and H4. Genetic and biochemical data have demonstrated that histone acetyltransferases such as yGcn5p and yEsa1p are recruited to the promoters of specific genes for the activation of transcription (2-6). Loss of function mutations of the genes encoding these enzymes, however, reveal that they are also responsible for a broad pattern of acetylation with relatively modest effects on the level of transcription (2, 4, 6).In Drosophila, evidence for special roles for particular histone modifications was revealed with antisera to specific isoforms of H4. While H4 isoforms acetylated at lysine 5 or 8 are associated with numerous sites throughout the genome, H4 acetylated at lysine 12 is enriched in chromocentric heterochromatin, and H4 acetylated at lysine 16 (H4Ac16) is exclusively associated with the male X chromosome (7, 8). Male-specific acetylation of H4 at lysine 16 is mediated by males absent on the first (MOF), 1 a MYST-family histone acetyltransferase present in the male-specific lethal (MSL) complex (9 -11). This histone acetyltransferase-containing complex is responsible for the dosage compensation of many X-linked genes by increasing their transcription in males to achieve a level of gene product equivalent to that generated by the two X chromosomes in females. In addition to its protein subunits, the MSL complex contains one of two nontranslated RNAs, roX1 and roX2 (11-15). Current models suggest that functional complexes form at the sites of transcription of these RNAs, then access the X chromosome at a small number of additional entry sites and subseq...

Chromosome-wide gene-specific targeting of theDrosophiladosage compensation complex

“…Our observation that there is no more polymerase on the X chromosome than autosomal sequences is consistent with previous suggestions that the DCC does not increase the amount of polymerase loading or promoter clearance, but rather the speed at which a transcript is completed (i.e., transcription elongation) (Smith et al 2001). Our detection of negligible amounts of DCC on promoters further supports this conclusion.…”

“…Pausing has been noted for all RNA polymerases, and is exacerbated on chromatin templates (for review, see Sims et al 2004). The acetylation of H4K16 in the middle and 3Ј end of two X-chromosomal genes suggests that H4K16 acetylation may follow a similar pattern to the DCC within genes themselves (Smith et al 2001). We have shown that H4K16 acetylation is very similar to the binding of the DCC at intermediate, restriction fragment resolution.…”

“…The DCC may aid transcription elongation rather than promoter accessibility, as H4K16 acetylation was found by chromatin IP in the middle and 3Ј end of two compensated genes rather than at the promoter (Smith et al 2001). In addition, on polytene chromosomes, Sass et al (2003) were able to discern MSL1 binding to the transcribed portion of a gene from a GAL4 activator presumably binding to the promoter.…”

roXRNAs Are Required for Increased Expression of X-Linked Genes inDrosophila melanogasterMales

“…Intriguingly, run is one of the X-linked genes with SXL-binding sites. The run gene does not accumulate H4Ac16 in males, and run expression is not decreased in roX1 À roX2 À or msl males (supplemental Tables S4 and S5 at http://www.genetics.org/supplemental/; Smith et al 2001). While this suggests that run is likely to be compensated in females by SXL, about half of the Xlinked genes that display multiple SXL-binding sites do decrease in roX1 À roX2 À males, indicating at least a partial regulation by the MSL complex (data not shown).…”

A male‐specific effect of dominant‐negative Fos

“…This complex assembles only in males because msl2 RNA is not translated in females (Kelley et al, 1997). Acetylation of nucleosomal histones on the male X chromosome is thought to facilitate transcriptional elongation approximately twofold (Smith et al, 2001). We investigated genetic interactions between the msl and mle mutations, and vg-Fbz, which shows a male-limited wing phenotype.…”