In the animal kingdom, with some exceptions, males and females of the same species are characterized by the type and number of their sex chromosomes. When females have two X chromosomes and they are termed homogametic (XX) whereas males are heterogametic with an X and a distinct Y (XY).
There is a non-trivial problem that arises from having a divergent pair of sex chromosomes between heterogametic vs. homogametic individuals. Having only one X, males would produce half the amount of X-linked gene products relative to females. This would be detrimental to the male since there are hundreds of essential genes on the X. Thus, under the pressure of natural selection, each species has independently come up with a solution to this imbalance due to the X chromosome number between the sexes, termed dosage compensation. Even though the end result is the same, different organisms employ different methods to equalize X-linked gene expression between males and females. A common theme though is the ability to selectively distinguish the X from the autosomes via sex-specific factors and to epigenetically regulate gene expression of an entire chromosome through chromatin modifications.
In Drosophila, this is achieved by increasing the transcriptional output from the single X chromosome in males by 2-fold to equal the levels in females. This process requires at least 5 proteins and 2 non-coding roX (RNA on the X) RNA components, collectively termed as the MSL (male-specific lethal) complex. MSL complexes target hundreds of sites along the X chromosome in males to up-regulate transcription.
Although the exact mechanism of X chromosome targeting is unknown, two lines of evidence have led to a model in which MSL1 and MSL2 form a core complex that initially binds to a limited number of high affinity sites, incorporates the roX transcripts and recruits the remaining components, including MSL3, to spread in cis into the flanking chromatin to achieve the final binding pattern. First evidence is that, in the absence of MSL3 (male-specific lethal 3), partial complexes can still bind an estimated ∼60-70 sites detected on the male polytene X chromosome, two of which are the roX1 and roX2 genes. In the absence of MSL1 or MSL2 however, binding to the X is completely lost. Second, although normally located on the X chromosome, a roX transgene inserted on an autosome is capable of attracting functional complexes and inducing MSL spreading into flanking regions, which are normally not dosage-compensated.
According to this model, MSL3 is thought to be involved in the spreading of the complex rather than initial nucleation at the high affinity sites. Since MSL3 is a chromo domain protein, one possible biochemical function could be to facilitate spreading by binding modified nucleosomes, as has been proposed for heterochromatin protein-1 (HP1) to facilitate assembly of heterochromatin. To test this, I have used mutants of MSL3 to study its role in complex spreading.
I found that even though the chromo domain is dispensable for initial targeting to the X, it is required for fine-tuning of MSL complex binding to achieve the correct final pattern. Using genome-wide analysis of binding sites for the mutant MSL3, I have found evidence that supports the spreading model; the first step in targeting is the binding of MSL complex to high affinity sites on the X and this is independent of MSL3. The second step involves chromo domain-dependent spreading from the high affinity sites to the majority of genes on the X to achieve the final binding pattern. Either deleting or introducing point mutations in the CD disrupts this latter step. The spreading step could be mediated by recognition of the histone H3 lysine 36 methylation mark on target genes via the MSL3 chromo domain, as chromo domain mutant MSL3 shows decreased binding to this modification in vitro.
My studies have additional findings that suggest that the current spreading model should be modified. I found evidence that more high affinity sites may exist on the X than estimated previously by polytene chromosome staining. These sites are only visible at the molecular level and they may share a common sequence element for targeting the complex independent of the MSL3 chromo domain. Further studies are required to show the requirement/contribution of specific sequences to targeting of these high affinity sites.