Cold-temperature induction of Escherichia coli polynucleotide phosphorylase occurs by reversal of its autoregulation

Abstract

When Escherichia coli cells are shifted to low temperatures (e.g. 15°C), growth halts while the 'cold shock response' (CSR) genes are induced, after which growth resumes. One CSR gene, pnp, encodes polynucleotide phosphorylase (PNPase), a 3′-exoribonuclease and component of the RNA degradosome. At 37°C, ribonuclease III (RNase III, encoded by rnc) cleaves the pnp untranslated leader, whereupon PNPase represses its own translation by an unknown mechanism. Here, we show that PNPase cold-temperature induction involves several post-transcriptional events, all of which require the intact pnp mRNA leader. The bulk of induction results from reversal of autoregulation at a step subsequent to RNase III cleavage of the pnp leader. We also found that pnp translation occurs throughout cold-temperature adaptation, whereas lacZ+ translation was delayed. This difference is striking, as both mRNAs are greatly stabilized upon the shift to 15°C. However, unlike the lacZ+ mRNA, which remains stable during adaptation, pnp mRNA decay accelerates. Together with other evidence, these results suggest that mRNA is generally stabilized upon a shift to cold temperatures, but that a CSR mRNA-specific decay process is initiated during adaptation.