ROLES OF RNASE-E, RNASE-II AND PNPASE IN THE DEGRADATION OF THE RPSO TRANSCRIPTS OF ESCHERICHIA-COLI - STABILIZING FUNCTION OF RNASE-II ANDEVIDENCE FOR EFFICIENT DEGRADATION IN AN AMS-PNP-RNB MUTANT

The Escherichia coli rpsO gene gives rise to different mRNA species re sulting either from termination of transcription or from processing of primary transcripts by RNase E and RNase III. The main degradation pa thway of these transcripts involves a rate-limiting RNase E cleavage d ownstream of the structural gene which removes the 3' terminal stem-lo op structure of the transcription terminator. This structure protects the message from the attack of 3'-5' exonucleases and its removal resu lts in very rapid degradation of the transcript by polynucleotide phos phorylase and RNase II. Polynucleotide phosphorylase is also able to d egrade slowly the mRNA harboring the 3' terminal hairpin of the termin ator. Tn contrast, RNase II appears to protect the rpsO mRNA species w hich retains the 3' hairpin structure. Rapid degradation of the rpsO m RNA is observed after inactivation of RNase II even in a strain defici ent for RNase E and polynucleotide phosphorylase. The enzyme(s) involv ed in this degradation pathway is not known. We detected an unstable e longated rpsO mRNA presumably resulting from the addition of nucleotid es at the 3' end of the transcript.