RNase III-Independent Autogenous Regulation of Escherichia coli Polynucleotide Phosphorylase via Translational Repression

The complex posttranscriptional regulation mechanism of the Escherichia coli pnp gene, which encodes the phosphorolytic exoribonuclease polynucleotide phosphorylase (PNPase), involves two endoribonucleases, namely, RNase III and RNase E, and PNPase itself, which thus autoregulates its own expression. The models proposed for pnp autoregulation posit that the target of PNPase is a mature pnp mRNA previously processed at its 5' end by RNase III, rather than the primary pnp transcript (RNase III-dependent models), and that PNPase activity eventually leads to pnp mRNA degradation by RNase E. However, some published data suggest that pnp expression may also be regulated through a PNPase-dependent, RNase III-independent mechanism. To address this issue, we constructed isogenic Δpnp rnc(+) and Δpnp Δrnc strains with a chromosomal pnp-lacZ translational fusion and measured β-galactosidase activity in the absence and presence of PNPase expressed by a plasmid. Our results show that PNPase also regulates its own expression via a reversible RNase III-independent pathway acting upstream from the RNase III-dependent branch. This pathway requires the PNPase RNA binding domains KH and S1 but not its phosphorolytic activity. We suggest that the RNase III-independent autoregulation of PNPase occurs at the level of translational repression, possibly by competition for pnp primary transcript between PNPase and the ribosomal protein S1.