Increased expression of Escherichia coli polynucleotide phosphorylase at low temperatures is linked to a decrease in the efficiency of autocontrol

Polynucleotide phosphorylase (PNPase) synthesis is translationally autocont rolled via an RNase III-dependent mechanism, which results in a tight corre lation between protein level and messenger stability. In cells grown at 18 degreesC, the amount of PNPase is twice that found in cells grown at 30 deg reesC. To investigate whether this effect was transcriptional or posttransc riptional, the expression of a set of pnp-lacZ transcriptional and translat ional fusions was analyzed in cells grown at different temperatures. In the absence of PNPase, there was no increase in pnp-lacZ expression, indicatin g that the increase in pnp expression occurs at a posttranscriptional level . Other experiments clearly show that increased pnp expression at low tempe rature is only observed under conditions in which the autocontrol mechanism of PNPase is functional. At low temperature, the destabilizing effect of P NPase on its own mRNA is less efficient, leading to a decrease in repressio n and an increase in the expression level.