MESSENGER-RNA DEGRADATION IN ESCHERICHIA-COLI - A NOVEL FACTOR WHICH IMPEDES THE EXORIBONUCLEOLYTIC ACTIVITY OF PNPASE AT STEM-LOOP STRUCTURES

Stem-loop structures can protect upstream mRNA from degradation by imp eding the processive activities of 3'-5' exoribonucleases. The ability of such structures to impede exonuclease activity in vitro is insuffi cient to account for the stability they can confer on mRNA in vivo. In this study we identify a factor from Escherichia coil which specifica lly impedes the processive activity of the 3'-5' exonuclease PNPase at stem-loop structures in vitro. This factor can, potentially, reconcil e the apparent. discrepancy between the ability of 3' stem-loop struct ures to stabilize upstream mRNA in vitro and in vivo. Its mechanism of action, and possible role in regulating mRNA degradation, is discusse d.