Dosage compensation strategies. Different organisms have evolved distinct dosage compensation strategies. In Drosophila, expression from the single X chromosome present in males is upregulated ∼twofold. In mammals, gene expression from one of the two X chromosomes present in females is silenced (a process also known as X-inactivation), whereas in the C. elegans XX sex, expression from both X chromosomes is halved. In mammals and C. elegans, a twofold upregulation of the X chromosomes in both sexes probably accompanies repression in XX animals to balance transcription between the X chromosome(s) and autosomes. Upregulated chromosomes are in green; chromosomes subject to transcriptional enhancement and repression are in yellow; the letter `A' denotes autosomes.

MSL complex components. (A) The MSL1 protein serves as the scaffold for the MSL complex, and its N-terminus is required for chromatin entry site (CES) recognition (Scott et al., 2000; Morales et al., 2004; Li et al., 2005). The coiled coil (CC) domain and PEHE domain (named for its characteristic amino acid composition) are shown (Marin, 2003). (B) The RING finger motif of the MSL2 protein is involved in interactions with MSL1, whereas its C-terminus (including a proline-rich domain) is required for the efficient incorporation of RNA on the X (roX) RNAs into the MSL complex (Copps et al., 1998; Li et al., 2008). MSL2 also contains a conserved cysteine-rich CXC domain of unknown function (Marin, 2003). (C) MSL3 interacts with MSL1 through its MRG domain, whereas its chromodomain (CD) is implicated in binding nucleosomes (nucs), nucleic acids and H3K36me3 (Morales et al., 2005; Buscaino et al., 2006; Sural et al., 2008). (D) The histone acetyltransferase Males absent on the first (MOF) and MSL3 interact with RNA in vitro (Akhtar et al., 2000). The histone acetyltransferase (HAT) domain of MOF is adjacent to a zinc-finger (ZF) domain that is required for MSL1 interaction and nucleosome binding (Akhtar and Becker, 2000; Smith et al., 2000; Akhtar and Becker, 2001; Morales et al., 2004). (E) Maleless (MLE) is an RNA/DNA helicase with multiple RNA-interaction domains, including its N-terminal double-stranded RNA-binding (RB) motifs and its C-terminal glycine-rich domain (Lee et al., 1997; Izzo et al., 2008). (F,G) The roX box elements (red) and inverse roX boxes (blue) are proposed to mediate the formation of alternative secondary structures in the non-coding RNAs roX1 and roX2 (Park et al., 2007; Kelley et al., 2008; Park et al., 2008).

Localization of the MSL complex on the X chromosome. Drosophila polytene chromosomes from the salivary glands of third instar larvae, labeled with an antibody specific for male-specific lethal (MSL) 2 (red) and with Hoechst to stain DNA (blue). (A) The MSL complex component MSL2 localizes specifically to the X chromosome (X) in wild-type males. (B) Ectopically expressed MSL2 recognizes a subset of sites in msl3 mutant females. msl3 mutant male larvae display a similar pattern, but are too unhealthy for optimal cytology. Images courtesy of Art Alekseyenko and Andrey Gortchakov.

Two-step model for MSL complex targeting. (A) The MSL complex specifically recognizes the X chromosome (X), not autosomes (Au), by co-transcriptional assembly at roX genes (blue) and by recognition of MSL recognition elements (MREs) at chromatin entry sites (CESs, red). Dashed lines indicate the possibility that roX RNAs are assembled into the MSL complex before the complex targets other CESs. (B) Spreading from these sites requires H3K36me3 (stars) and perhaps other features of active genes. It has been demonstrated that the MSL complex can recognize active genes (tan) that lack X chromosome-specific sequences. However, there is evidence that MSL binding might be reinforced in some cases by the presence of additional sequence elements (green bars). Dashed lines indicate hypothetical paths of MSL spreading, although other paths are possible. Spreading might involve a linear scanning mechanism, or it may reflect a capture-and-release mechanism in three-dimensional space. In either case, the MSL complex is able to `skip over' inactive genes (gray).