Functional analysis of the long terminal repeats of Drosophila 1731 retrotransposon: promoter function and steroid regulation.

1731 is a Drosophila retrotransposon whose transcripts decrease in Drosophila cells after treatment by the steroid hormone 20-hydroxyecdysone (20-OH). Several constructions have been made where the bacterial chloramphenicol acetyltransferase (CAT) gene is put under the control of either the 5' or the 3' long terminal repeats (LTRs) of 1731. CAT activity assays in transfected Drosophila cells show that either the 5' or the 3'LTR constitutes a unidirectional promoter. Analysis of partially deleted LTR suggests the presence of so-called silencer and activator regions in these LTRs. Moreover, the first 260 bp of the LTR are sufficient to provoke 20-OH inhibition whereas the first 58 bp are necessary for hormonal responsiveness. These 58 bp contain sequences showing similarities with the targets of trans-acting factors such as Octal-c and NFkB.