Transcriptional and post-transcriptional control of polynucleotide phosphorylase during cold acclimation in Escherichia coli

Polynucleotide phosphorylase (PNPase, polyribonucleotide nucleotidyltransfe rase, EC 2.7.7.8) is one of the cold shock-induced proteins in Escherichia coli and pnp, the gene encoding it, is essential for growth at low temperat ures. We have analysed the expression of pnp upon cold shock and found a dr amatic transient variation of pnp transcription profile: within the first h our after temperature downshift the amount of pnp transcripts detectable by Northern blotting increased more than 10-fold and new mRNA species that co ver pnp and the downstream region, including the cold shock gene deaD, appe ared; 2 h after temperature downshift the transcription profile reverted to a preshift-like pattern in a PNPase-independent manner. The higher amount of pnp transcripts appeared to be mainly due to an increased stability of t he RNAs. The abundance of pnp transcripts was not paralleled by comparable variation of the protein: PNPase steadily increased about twofold during th e first 3 h at low temperature, as determined both by Western blotting and enzymatic activity assay, suggesting that PNPase, unlike other known cold s hock proteins, is not efficiently translated in the acclimation phase. In e xperiments aimed at assessing the role of PNPase in autogenous control duri ng cold shock, we detected a Rho-dependent termination site within pnp. In the cold acclimation phase, termination at this site depended upon the pres ence of PNPase, suggesting that during cold shock pnp is autogenously regul ated at the level of transcription elongation.