EXPRESSION OF GENE-19 OF THE CONJUGATIVE PLASMID R1 IS CONTROLLED BY RNASE-III

Specific cleavage of mRNAs by RNase III has been shown to control the expression of several Escherichia coli genes. We show here that the ex pression of gene 19 of the conjugative resistance plasmid R1 is contro lled in its expression by the same endoribonuclease. In vivo studies r evealed that a DNA fragment of 150 nucleotides including a perfect 22 nucleotide inverted repeat in the gene 19 coding region is responsible for the low expression of the gene both at the protein and the RNA le vels. By using a translational gene 19-lacZ fusion in isogenic RNase I II+ and RNase III- strains we could identify RNase III as the key elem ent in the down-regulation of gene 19 expression. The sequencing of in vitro generated and RNase III-digested transcripts confirmed the in v ivo studies and revealed the exact positions of the RNase III cleavage sites within the coding part of the gene 19 transcript. The in vitro determined RNase III cleavage of gene 19 mRNA was confirmed by in vivo primer extension analysis. Finally, we could show that an exchange of three nucleotides within the RNase III recognition site abolished RNa se III cleavage in vitro.