RNASE-III CLEAVAGES IN NONCODING LEADERS OF ESCHERICHIA-COLI TRANSCRIPTS CONTROL MESSENGER-RNA STABILITY AND GENETIC EXPRESSION

The primary transcripts of the rpsO-pnp, rnc-era-recO and metY-nusA-infB operons of E coli are each processed by RNase III, upstream of the first translated gene, in hair-pin structures formed by the 5' non-coding leader. The mRNAs of the 3 operons, of which the 5' terminal motifs have been removed by RNase III, decay significantly more rapidly than the uncut transcripts which accumulate in the RNase III deficient strain. The rapid decay of a primary transcript of the metY-nusA-infB operon, initiated at a secondary promoter in the vicinity of the RNase III sites, suggests that the 5' features upstream of the RNase III cutting sites are responsible for the stability of the uncut RNAs. RNase III autocontrols its own expression by removing the 5' motif which stabilizes its mRNA. Similarly, the synthesis of polynucleotide phosphorylase and of protein Era are also controlled by RNase III cleavages which trigger the degradation of their messengers. The role of RNase III in the regulation of gene expression and the possible mechanisms of mRNA stabilization and of 5' to 3' decay initiated by RNase III processing are discussed.