Akhtar Group (Visiting)
Mechanisms of transcription regulation through chromatin
Immunostaining of polytene chromosomes from salivary glands of male Drosophila using antibodies directed against members of the dosage compensation complex (DCC). The figure shows that MSL-3 and MSL-2 co-localise specifically on hundreds of sites on the male X chromosome. All the chromosomes are also stained with Hoechst to show staining of DNA. The position of the X chromosome is indicated by X.
Previous and current research
DNA tightly packed together with histones into nucleosomes is not easily accessible to the enzymes that use it as a template for transcription or replication. Consequently, remodelling of chromatin structure may play an essential role in the regulation of gene expression. Structural changes in chromatin may also form the basis for dosage compensation mechanisms that have evolved to equalise levels of X-linked gene products between males and females. In humans, one of the two X chromosomes in females is randomly inactivated by condensation of the chromosome into a Barr body, a process known as X-inactivation. In contrast, in Drosophila this is achieved by a two fold hyper-transcription of the genes on the male X chromosome. Genetic studies have identified a number of factors that are important for dosage compensation in Drosophila, including five proteins (MSL1, MSL2, MSL3, MLE, MOF) and two non-coding RNAs (roX1 and roX2). The hyperactive X is also specifically hyper-acetylated at histone H4, acetylation which is achieved by the MOF histone acetyl transferase.
Our major goal is to study the epigenetic mechanisms underlying X-chromosome specific gene regulation using Drosophila dosage compensation as a model system. More specifically, we are interested in addressing how the dosage compensation complex, composed of RNA and proteins (the MSL complex), gets targeted to the X chromosome. In addition, we are studying the mechanism by which the MSL complex modulates X chromosomal transcriptional output.
Future projects and goals
The role of nuclear periphery in X chromosomal regulation.We have recently discovered the involvement of nuclear pore components in the regulation of dosage compensation in Drosophila. This work has raised several interesting questions about the role of genome organisation and gene regulation, which we will continue to actively address in the future. In addition to using functional genomic approaches, we plan to study in detail the mechanism of nuclear pore/X chromosomal interaction by employing detail cell biology and biochemical chromatin based strategies. This multifaceted approach will be instrumental in future studies to decipher the mechanism of X chromosomal regulation by the MSL complex.
The role of non-coding RNA in dosage compensation. The involvement of non-coding RNAs as potential targeting molecules adds another level of complexity to chromatin regulation. Interestingly, the dosage compensation complex includes two non-coding roX RNAs. However, the mechanism by which these RNAs function is unknown. One of our future aims will be to elucidate how these interactions influence transcription activation of the X-linked genes.
The function of the mammalian MSL complex. There is a remarkable evolutionary conservation of all the known Drosophila dosage compensation complex members in mammals. In fact, we have recently purified the Drosophila and mammalian MSL complexes and shown that there is a high degree of conservation also at the biochemical level, implying a functional role for the mammalian MSL complex in gene regulation which we will continue to study.