Post-Transcriptional Control of IS10 Transposase Expression: Antisense RNA Binding and Other Conformational Changes Affecting Messenger RNA Stability and Translation

Abstract

Insertion sequence IS10 (Fig. 1) encodes a transposase function (tnp) that is rate-limiting for transposition (Morisato et al., 1983). Transposase expression is controlled at both the transcriptional and post-transcriptional levels. The tnp promoter is among the weakest of well-characterized promoters (Simons et al., 1983; Case et al., 1988) and its activity is inhibited by DNA-adenine methylation (Roberts et al., 1985) and integration host factor binding (J. Krull and R.W.S., in preparation). Here, we are concerned with three different post-transcriptional mechanisms that operate to control tnp translation (Fig. 1). All three manifest their effects by altering RNA secondary structure at or near the tnp ribosome binding site. First, a small antisense RNA (Fig. 1B) binds to the 5′ end of short (nascent) the tnp mRNAs (Fig. 1A) to block ribosome binding (Fig 1C). Second, RNA secondary structure in the full-length tnp mRNA sequesters the 5′ end (Fig. 1D), resulting in two effects: insensitivity to antisense RNA binding and decreased tnp translation. Third, tnp mRNAs initiated at external promoters (Fig. 1E) are not translated efficiently because RNA secondary structure sequesters the tnp ribosome binding site. The salient features of each control mechanism are described below.