PNPase activity is modulated (at least in vitro) by cyclic-di-GMP [63], a signal molecule

implicated in biofilm formation [18]. However, deletion of the dos gene, encoding a c-di-GMP phosphodiesterase which co-purifies with the RNA degradosome [63], did not affect pgaABCD expression (data not shown). Key molecules in energy metabolism and carbon flux, such as ATP and citrate also influence PNPase activity [64, 65]. Thus, it can be speculated that environmental or physiological signals might regulate pgaABCD expression by controlling the level of specific metabolites that could directly modulate PNPase activity. Our data clearly indicate that PNPase controls PNAG production by negatively regulating the pgaABCD operon at post-transcriptional level and that it targets the 5’-UTR of the pgaABCD transcript, thus similar to the translational Geneticin chemical structure repressor CsrA (Figures 4 5 and Additional file 4: Figure CP673451 S3). This would suggest that the two proteins might belong to the same regulatory network. However, probing this hypothesis is complicated by the observation that in E. coli C, the

mechanisms of CsrA-dependent gene expression regulation and its modulation by small RNAs might be more complex than in E. coli K-12, where the current model for CsrA regulation has been developed. This notion is somehow suggested by the fact that, while deletion of the csrA gene is lethal for E. coli K-12 when grown on glucose-based media [55], this is not the case for E. coli C. Moreover, to our surprise,

the lack of putative positive regulators such as CsrB, CsrC and McsA resulted in an increase of pgaABCD expression levels both in the Δpnp and in its parental strain C-1a, which would suggest a negative role of these sRNAs in pgaABCD control (Figure 5). Genes encoding cell surface-associated structures seem to constitute a “hotspot” for post-transcriptional regulation involving small non coding RNAs. For instance, multiple control of gene expression by sRNAs has already been demonstrated for csgD, which encodes the master Peptide 17 ic50 regulator for the biosynthesis of thin aggregative fimbriae (curli), one of the major adhesion factors in E. coli[28, 55, 66, 67]. It is thus possible that, in E. coli C, increased pgaABCD expression in mutant strains carrying deletions Temsirolimus chemical structure of sRNA-encoding genes might be due to feedback induction of yet unidentified factors which might play a role in CsrA-dependent regulation. This possibility is supported by the observation that CsrB, CsrC and McaS mutually control their transcript level both in E. coli K and C [53] (T. Carzaniga and F. Briani, unpublished data). pgaABCD operon regulation appears to be an intriguing model system for the study of post-transcriptional modulation of gene expression in bacteria. Conclusions In this work, we have unravelled a novel role for PNPase as a negative regulator of pgaABCD expression and PNAG biosynthesis. Thus, PNPase activity contributes to keeping E.