Interdependence of translation, transcription and mRNA degradation in the lacZ gene.

We have constructed a collection of Escherichia coli strains which differ by point mutations in the ribosome binding site (RBS) that drives the translation of the lacZ gene. These mutations affect the Shine-Dalgarno sequence or the initiation codon, or create secondary structures that sequester these elements, and result in a 200-fold variation in beta-galactosidase expression. Surprisingly, these variations of expression are paralleled by nearly equivalent changes in the lacZ mRNA level. The ratio of the beta-galactosidase expression to the mRNA level reflects the average spacing between translating ribosomes: hence, paradoxically, mutations that affect translation initiation do not correspondingly change this spacing. Further analysis of the mRNA level variations shows that they originate from two independent mechanisms. When beta-galactosidase expression exceeds a threshold corresponding roughly to one translation event per transcript, the variations in the efficiency of translation initiation affect largely the chemical and functional lifetimes of the mRNA. We further show that the rate-limiting step in the chemical decay process is an RNase E-dependent cleavage, which is outcompeted by translation initiation. Below this expression threshold, the mRNA lifetime levels out and strain-to-strain variations in mRNA level arise solely from polarity effects. We suggest that, in this activity range, most mRNA molecules that escape polarity are crossed by a single ribosome, and hence are identical from the viewpoint of degradation. Altogether, the tight couplings between translation initiation on one hand, polarity and/or mRNA degradation on the other, result in translation initiation events being closely spaced in time even from inefficient RBS, at the expense of the mRNA level. Finally, we evocate the possible beneficial consequences of a coupling between translation, transcription and mRNA degradation, for the management of cellular resources.