Correlation of translation efficiency with the decay of lacZ mRNA in Escherichia coli.

We isolated mutations in the leader of a ribosomal protein (r-protein)/lacZ fusion gene in Escherichia coli that caused the mRNA to be translated at efficiencies between < 1% and 62% of the rate of wild-type message. Using a subset of these mutants with translation efficiencies between 5% and 62%, we studied both physical and functional decay of the mRNA after rifampicin inhibition of transcription initiation. The decay of physically intact transcript was analyzed by gel electrophoresis of hybrid-selected messenger RNA segments. The output from the message was analyzed by measuring the synthesis rate of r-protein/lacZ fusion protein. Decay of physically intact message after rifampicin addition correlated with the translation efficiency, with the more active messengers being more stable. Different segments of the r-protein/lacZ fusion mRNA decayed with the same rate, indicating that there is no hyper-labile region in the transcript. The decay rate was also independent of the length of the segment probed, suggesting that the mRNA is not degraded by random attacks along the entire length of the molecule. Our results are consistent with an overall 5' to 3' degradation pathway. Surprisingly, the rate of fusion protein synthesis did not decrease immediately after rifampicin addition. Rather, a lag preceded the exponential decay phase; the length of this delay correlated with the translation efficiency, such that the lag increased with increasing efficiency of translation. We suggest that these lags indicate that mRNAs are normally competing for ribosomes during exponential growth and, after rifampicin addition, RNA molecules with longer physical half-lives are translated by ribosomes released from fast decaying messengers.