Evidence for endonucleolytic cleavages in decay of lacZ and lacI mRNAs

S1 nuclease mapping revealed lacZ mRNA molecules whose 5' and 3' ends were internal to the transcription start and consistent with cleavages at pyrimidine-adenosine bonds 20 to 50 nucleotides apart. With the net 5'-to-3' direction known, lacZ mRNA is probably degraded by sequential cleavages of naked mRNA at vulnerable sites exposed by transit of the last translating ribosome.