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

When Escherichia coli cells are shifted to low temperatures (e.g. 15 degree sC), growth halts while the 'cold shock response' (CSR) genes are induced, after which growth resumes. One CSR gene, pnp, encodes polynucleotide phosp horylase (PNPase), a 3'-exoribonuclease and component of the RNA degradosom e. At 37 degreesC, 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 int act pnp mRNA leader. The bulk of induction results from reversal of autoreg ulation at a step subsequent to RNase III cleavage of the pnp leader. We al so 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 degreesC. However, u nlike the lacZ(+) mRNA, which remains stable during adaptation, pnp mRNA de cay accelerates. Together with other evidence, these results suggest that m RNA is generally stabilized upon a shift to cold temperatures, but that a C SR mRNA-specific decay process is initiated during adaptation.