In the Escherichia coli lacZ gene the spacing between the translating ribosomes is insensitive to the efficiency of translation initiation.

We have constructed a series of 44 Escherichia coli strains in which the chromosomal region corresponding to the Ribosome Binding Site (RBS) of the lacZ gene, has been replaced by small DNA fragments harboring either RBSs from other genes, or artificial RBSs. The beta-galactosidase expression from these strains ranges from 1 to 130 per cent of that of the parental strain. Using this collection, we demonstrate here that strain-to-strain variations in expression are paralleled by nearly equivalent variations in lacZ mRNA content. We propose that, in this system, polarity and mRNA stability are tightly coupled to translation initiation, so that changes in RBS efficiency are detected mainly as changes in mRNA concentration rather than in the spacing between translating ribosomes. In addition, we show that the mRNA sequence immediately downstream from the initiator codon influences per se the lifetime of the lacZ mRNA. We discuss the mechanism of the interdependence between translation, transcription and degradation in this gene, and speculate about the general role of this interdependence in determining the expression of bacterial genes.