Transcriptional regulation of the mosquito vitellogenin gene via a blood meal-triggered cascade.

In anautogenous mosquitoes, a blood meal is required for activation of genes encoding yolk protein precursors (YPP). Vitellogenin (Vg), the major YPP gene, is transcribed at a very high level following blood meal activation. It is expressed exclusively in the female fat body, the tissue producing most of mosquito hemolymph and immune proteins. In this paper, we analyzed the upstream region of the Aedes aegypti Vg gene in order to identify regulatory elements responsible for its unique expression pattern. To achieve this goal, we analyzed the gene using transgenic Drosophila and Aedes as well as DNA-binding assays. These analyses revealed three regulatory regions in the 2.1 kb upstream portion of the Vg gene. The proximal region containing binding sites to EcR/USP, GATA, C/EBP and HNF3/fkh is required for the correct tissue- and stage-specific expression at a low level. The median region carrying sites for early ecdysone response factors E74 and E75 is responsible for hormonal enhancement of Vg expression. Finally, the distal GATA-rich region is necessary for extremely high expression levels characteristic of the Vg gene. The present work elucidates the molecular basis of blood meal-dependent expression of this mosquito gene, laying the foundation for mosquito-specific expression cassettes with predictable stage and tissue specificity.