Molecular and genetic studies of the E75 early gene in Drosophila

The steroid hormone, 20-hydroxyecdysone (20E), is responsible for initiating major developmental transitions in the fruit fly Drosophila melanogaster. Increases in the hormone titer induce a genetic cascade specific for the stage and tissue in which the increase occurs. Insights into these 20E induced genetic cascades came from in vivo and in vitro studies of polytene chromosome puffing in late third instar larval salivary glands. It was found that 20E directly induces the appearance of a small set of early puffs several hours before pupariation. Upon puparium formation, these early puffs begin to regress and are replaced by a set of over 100 late puffs that appear and regress in a specific temporal order. A molecular model to explain these results was proposed. It states that 20E binds its receptor, and this 20E/receptor complex directly induces the expression of the early genes. These genes encode regulatory proteins that repress early genes and induce the expression of the late genes. Late genes encode proteins that have a direct effect on the biological process necessary for the developmental stage and tissue in which the 20E pulse occurred. To test this model and to further understand how a hormone signal is transduced into a biological response, genes responsible for early and late puffs must be identified and characterized. This thesis presents an analysis of loss-of-function mutants for E75, which encodes three related members of the nuclear receptor superfamily and is responsible for the 75B early puff. Mutants in which the entire E75 locus has been disrupted hatch to the first larval instar, but die up to one week later without molting. E75B mutants are viable and show no detectable phenotypes. E75C mutants have eye and behavioral defects and die several days after eclosing. E75A mutants die throughout development but never eclose. They display phenotypes characteristic of 20E deficient mutants. Indeed, the 20E titer is reduced at least 10-fold in E75A mutant larvae. A model is proposed that states E75A is involved in a feed-forward mechanism to amplify or extend an initial 20E pulse by inducing the expression of genes that encode steroidogenic enzymes.