Propionate is an abundant carboxylic acid in nature. Microorganisms metabolize propionate aerobically via the 2-methylcitrate pathway. This pathway depends on a series of three reactions in the citric acid cycle that leads to the conversion of succinate to oxaloacetate. Interestingly, the γ-proteobacterium Escherichia coli can use propionate as a carbon and electron source under oxic but not under anoxic conditions. RT-PCR and transcriptomic analysis revealed a posttranscriptional regulation of the prpBCDE-gene cluster encoding the necessary enzymes for propionate metabolism. The polycistronic mRNA seems to be hydrolyzed in the 3′–5′ direction under anoxic conditions. This regulatory strategy is highly constructive because the last gene of the operon encodes the first enzyme of the propionate metabolism. Further analysis revealed that RNase R is involved in the hydrolysis of the prp transcripts. Consequently, an rnr-deletion strain could metabolize propionate under anoxic conditions. To the best of our knowledge, this is the first study describing the influence of RNase R on the anaerobic metabolism of E. coli.
