Intermediates in the degradation of mRNA from the lactose operon of Escherichia coli.

We have analyzed the processing of mRNA from the lac operon in an Escherichia coli strain carrying the lac on a multicopy plasmid. Messenger RNA was analyzed by hybridization and nuclease protection of pulse-labeled RNA and precursor-product relationships were determined by quantitating radioactivity in primary and processed transcripts at various times after induction of the lac promoter or inhibition of transcription with rifampicin. Our results support the existence of two types of processed transcripts with endpoints in the lacZ-lacY intercistronic region. One of these carries lacZ sequences and has a 3' endpoint about 30 bases downstream of this gene. The other carries lacY sequences and has a 5' end in the translation termination region of the lacZ gene. Finally, we have found evidence that transcription is continued at least 268 bases beyond the last gene (lacA) and that this 3' non-translated region is shortened by post-transcriptional processing.