Decay of ompA mRNA and processing of 9S RNA are immediately affected by shifts in growth rate, but in opposite manners.

By growing Escherichia coli in continuous cultures at various growth rates, we provide definitive evidence that the stability of the ompA mRNA is growth rate dependent. Shifting fast-growing cells into physiological salt buffer led to an immediately increased rate of ompA mRNA decay and to an instantly decreased rate of 9S RNA conversion into 5S rRNA. Shifting slowly growing cells into fresh medium had the opposite effect for each of the two RNA species. The observed regulatory patterns underline the need of cells to adjust the output of ompA and 9S RNAs in response to growth rate changes. At all growth rates and throughout all shift experiments, the half-life of bla mRNA was constant. A stabilization of the ompA transcript was even observed when slowly growing cells were shifted into fresh medium already containing the transcriptional inhibitor rifampicin. A hybrid bla transcript with the 5' untranslated region from the ompA gene behaved similarly to the wild-type ompA messenger in response to a shift in growth rate. In agreement with this result, we found that the same type of 5' cleavages as have been previously shown to initiate the decay of the ompA transcript seem to be involved in stability regulation. In E. coli the degradation of mRNA has been shown to depend on the ams/rne gene. This gene controls the stability-related cleavages in the ompA transcript, catabolic processes, and the cleavages which process the 9S rRNA into 5S RNA, an anabolic process. We discuss these results with respect to the ams/rne gene and the related nuclease activities that control the ompA and 9S RNA cleavages.