Extent of Chromatin Spreading Determined by roX RNA Recruitment of MSL Proteins

Abstract

The untranslated roX1 and roX2 RNAs are components of the Drosophila male-specific lethal (MSL) complex, which modifies histones to up-regulate transcription of the male X chromosome. roX genes are normally located on the X chromosome, and roX transgenes can misdirect the dosage compensation machinery to spread locally on other chromosomes. Here we define MSL protein abundance as a determinant of whether the MSL complex will spread in cis from an autosomal roX transgene. The number of expressed roX genes in a nucleus was inversely correlated with spreading from roX transgenes. We suggest a model in which MSL proteins assemble into active complexes by binding nascent roX transcripts. When MSL protein/roX RNA ratios are high, assembly will be efficient, and complexes may be completed while still tethered to the DNA template. We propose that this local production of MSL complexes determines the extent of spreading into flanking chromatin.

Dosage Compensation inDrosophila

“…Under specific genetic conditions (e.g., when there are no competing endogenous roX genes on the X chromosome), extensive spreading from autosomal roX transgenes is consistently seen (Fig. 5E,F) (Park et al 2002). This extensive spreading is augmented by overexpression of MSL1 and MSL2, the key limiting MSL proteins, and diminished by overexpression of roX RNA from competing transgenes, suggesting that successful cotranscriptional assembly of MSL complexes may drive local spreading (Oh et al 2003).…”

X-chromosome-wide profiling of MSL-1 distribution and dosage compensation inDrosophila

“…Indeed, MOF is able to relieve chromatin-mediated repression of transcription in vitro and in vivo when targeted to a promoter (Akhtar and Becker 2000). Moreover, targeting the DCC artificially on an autosome leads to up-regulation of the downstream gene (Henry et al 2001;Park et al 2002). However, the transcriptional activation performed by the MSL complex should not overcome individual gene regulation, linked to developmental pathways, but rather fine-tune the transcription to ensure dosage compensation.…”

The Drosophila Dosage Compensation Complex Binds to Polytene Chromosomes Independently of Developmental Changes in Transcription

“…It was shown earlier that roX RNAs, as well as histone acetyltransferase and ATPase activities of MOF and MLE, respectively, are needed for recruiting the DCC to multiple non-CES binding sites Park et al 2002). Recently, it was reported that the DCC-binding pattern on the male X reflects the distribution of genes, which are active in the tissue at this moment of development (Sass et al 2003).…”

“…Of hundreds of DCCbinding sites along the male X chromosome 70 associate with an incomplete complex (Lyman et al 1997;Gu et al 1998;Demakova et al 2003). These were proposed to be DNA targets serving as chromatin entry sites (CESs) and playing a key role both in recruitment of DCC to the X chromosome and in subsequent complex spreading to all additional sites (Lyman et al 1997;Kelley et al 1999;Park et al 2002). Further studies on DCC binding to the X under varying levels of MSL2 have led to the idea that this process is directed by a hierarchy of target sites displaying different affinities for the complex (Demakova et al 2003).…”