Control of gene expression occurs at many levels. It is now understood that regulation after transcription plays a crucial role in development and in the function of most if not all cell types.
A variety of post-transcriptional control mechanisms operate in early Drosophila development, and some of these play central and essential roles in the deployment of the spatial determinants that control body pattern. These determinants must be restricted to discrete regions of the oocyte or embryo. This restriction is acheived by a combination of mRNA localization, translational repression and activation, and protein anchoring. Projects in the lab have addressed each of these mechanisms, focusing primarily on the localization of bicoid mRNA, and on the localization and translational regulation of oskar mRNA.
Current projects continue the work on the post-transcriptional regulation of mRNAs encoding the patterning determinants. We are extending our analysis of the role of Bruno in regulation of osk and other mRNAs. We first identified Bruno as a repressor of osk mRNA translation, and subsequently found that the Bruno binding sites are also required for translational activation. Most our our work in this area addresses two topics. One is the question of how translation is activated, and how activation is coordinated with posterior localization of osk mRNA. The second topic concerns the phenomenon of translational regulation in trans. It is generally assumed that cis regulatory elements only act in cis, as their name implies. Surprisingly, however, we found that the Bruno binding sites in osk mRNA can act in trans, influencing the regulation of other molecules of osk mRNA.
In a related project we are examining the cellular context for the molecular interactions of post-transcriptional regulation. The binding of Bruno to osk mRNA does not occur in isolation, but takes place in the complex environment of the cytoplasm. There are several classes of large ribonucleoprotein complexes (RNPs) that are enriched in Bruno or other regulatory factors. These include sponge bodies (which are enriched near nuclei), nuage (a perinuclear organelle that may be common to all animal germline cells) and polar granules (which are localized to the posterior pole of the oocyte and contain the determinants for posterior body patterning and germ cell formation). These RNPs are presumed to be important for regulation, but details of their functions remain largely unknown. We suspect that regulation in trans is only possible because of close physical proximity of mRNAs.