The importance of the 5'-region in regulating the stability of sdh mRNA in Bacillus subtilis.

The decay of the polycistronic Bacillus subtilis sdh mRNA was analysed using probes specific for each of the component cistrons, sdhC, sdhA and sdhB. In exponentially growing cells, the entire sdh mRNA seems to decay with an 'all or nothing' mechanism and with a uniform half-life of 2-3 min for all cistrons. In stationary-phase cells, the half-life of the 5'-part had dropped to about 0.6 min whereas that of the 3'-part was about 1.2 min. Decay of sdh mRNA was also measured in exponentially growing cells containing a 'down-mutation' in the ribosomal binding site preceding sdhC which decreases the expression of sdhC by about 90%. The mutation has a moderate effect on expression of the downstream cistron sdhA. In this mutant, the half-life of the 5'-part of sdh mRNA was about 0.5 min (i.e. the same as in stationary phase wild-type cells) and the half-life of the 3'-part about 1.3 min. Also, analysis of the decay of an sdh-cat fusion transcript revealed that the sdh (5') part decayed more rapidly than the cat part and this difference was more pronounced in stationary-phase cells compared to exponentially growing cells. The results of these experiments demonstrate the importance of the 5'-segment of sdh mRNA in controlling the stability of the transcript under different growth conditions.