Regulatory RNA plays a pivotal role in the regulation of bacterial gene expression. Here, five small RNAs were studied in Sinorhizobium meliloti - SmrC15, SmrC16, Sra33, 6S and the signal recognition particle (SRP) RNA, which are conserved among at least seven different Rhizobium and Sinorhizobium species. The amount of SmrC16 decreased in stationary phase, while the other RNAs were Lip-regulated. The smallest changes, maximally 2-fold, were observed for SS RNA. In the distantly related Brodyrhizobium japonicum, the amount of SS RNA was not increased in stationary phase, suggesting some functional divergence in the roles of this molecule in Rhizobiales in comparison to Escherichia coli. Different decay rates were observed for SmrC15 and SmrC16 of S. meliloti upon rifampicin treatment, revealing posttranscriptional regulation during growth. The use of a Delta hfq Mutant showed that Hfq protects full-length SmrC16 from degradation and stabilises its specific degradation products. (C) 2009 Elsevier Inc. All rights reserved