PNPASE MODULATES RNASE-II EXPRESSION IN ESCHERICHIA-COLI - IMPLICATIONS FOR MESSENGER-RNA DECAY AND CELL-METABOLISM

PNPase and RNase II are the key regulatory exonucleases controlling mR NA decay in Escherichia coli. The mb transcripts were found to proceed through the terminator and PNPase was found to be involved in the 3' to 5' degradation of mb mRNA. Analysis of these longer 3' termini reve aled that they are located in UA-rich regions. Comparison of single an d double mutants suggested that PNPase and RNase II could have differe nt roles in the degradation of these unstructured regions. We have sho wn that RNase II levels can vary over a fivefold range in haploid cell s and that its expression depends on PNPase levels. PNPase-deficient s trains were found to have a 2-2.5-fold increase in RNase II activity, while PNPase-overproducing strains reduced the mb message and RNase II levels. Conversely, the amount of PNPase in the mb deletion strain wa s approximately twofold higher than that in the wild-type strain. Thes e observations suggest that the two main exonucleases are inter-regula ted through a fine tuning mechanism. We discuss the implications of th ese results with regard to mRNA degradation and cell metabolism.