THE STABILITY OF ESCHERICHIA-COLI LACZ MESSENGER-RNA DEPENDS UPON THESIMULTANEITY OF ITS SYNTHESIS AND TRANSLATION

We have used either Escherichia coli or T7 RNA polymerase to transcrib e in E.coli a series of lacZ genes that differ in the nature of their ribosome binding sites (RES). Each T7 RNA polymerase transcript yields from 15- to 450-fold less beta-galactosidase than its E.coli polymera se counterpart, the ratio being larger when weaker RBS are used. The l ow beta-galactosidase yield from T7 transcripts reflects their low sta bility: the ams-1/rne-50 mutation, which inactivates RNase E, nearly e qualizes the beta-galactosidase yields from T7 and E.coli RNA polymera se transcripts. T7 RNA polymerase transcribes the lacZ gene similar to 8-fold faster than the E.coli enzyme. We propose that this higher spe ed unmasks an RNase E cleavage site which is normally shielded by ribo somes soon after its synthesis when the slower E.coli enzyme is used. This leads to degradation of the T7 transcript, unless the leading rib osome comes in time to shield the cleavage site: the weaker the RBS, t he lower this probability and the more severe the inability of T7 RNA polymerase transcripts for beta-galactosidase synthesis.