Evidence for endonucleolytic attack in decay of lac messenger RNA in Escherichia coli.

Abstract The size distribution of decaying messenger RNA molecules from the lactose (lac) operon of Escherichia coli has been measured. A one-minute induction period was terminated by rifampicin, then a further period was allowed for completion of the lac mRNA transcription. All incomplete lac mRNA molecules could then be identified as degradation products. RNA was labeled with [3H]-uracil and purified by procedures which produced no significant cleavage. The purified RNA was centrifuged in sucrose gradients and then each fraction was hybridized with excess φ80dlac DNA to determine the amounts of lac message as a function of size. The size distributions observed at various times have been compared to those expected according to different mechanisms of degradation. These include a 5′ to 3′ exonucleolytic degradation as opposed to mechanisms in which the primary attack is endonucleolytic. The results clearly exclude an exclusive exonucleolytic degradation. Although the number and distribution of sites subject to endonucleolytic cleavage cannot be determined, the results are consistent with a model in which there are vulnerable sites at the start of the message specifying each polypeptide chain.