IS 10 mRNA stability and steady state levels in Escherichia coli: indirect effects of translation and role of rne function

Translation of the IS 10 transposase gene is known to be very infrequent. We have identified mutations whose genetic properties suggest that they act directly to increase or decrease the intrinsic level of translation initiation. Also, we have analysed in detail the effects of these mutations on IS 10 mRNA using one particular IS 10 derivative. In this case, increases or decreases in translation are accompanied by increases or decreases in both the steady state level and the half-life of transposase mRNA; effects on steady state levels are much more dramatic than effects on message half-life. At wild-type levels of translation initiation, the rate-limiting step in physical decay of full length IS 10 message for a particular IS 10 derivative is shown to be rne-dependent endonucleolytic cleavage; 3′ exonucleases appear to play a secondary role, degrading primary cleavage products. Analysis of interplay between translation mutations and me function, together with the above observations, suggests that translation stabilizes messages in a general way against rne-dependent endonucleolytic cleavage , and that significant protection may be conferred by one or a few ribosomes. However, dramatic effects of translation on steady state message levels are still observed in an me mutant and involve the 3′ end of the transcript; we propose that these additional effects reflect translation-mediated stimulation of transcript release.