Ribonuclease Activity in <i>Escherichia coli</i> Polyribosomes

Polyribosomes containing mRNA differentially labelled with [3H]uracil and [14C] uracil in the 5′‐ and the 3′‐proximal protions were isolated from Escherichia coli cells. mRNA degradation to alcohol‐soluble products occurred when polyribosomes were incubated in conditions of protein synthesis and proceeded in the 5′ to 3′‐direction. 15% of the mRNA was degraded at the end of a 10‐min period of incubation.The mRNA‐degrading enzyme (or enzymic system) was found to be bound to polyribosmes; it could be removed by washing in low salt. The content in RNAase activity per ribosomes in polyribosomes increased three to five‐fold when polyribosomes were isolated from cells in which extensive run‐off of ribosomes from mRNA had been induced. On the contrary the RNAase activity in free ribosomes was not affected. This is taken as an indication that the enzyme(s) responsible for the degradation of endogenous mRNA in vitro is not aspecifically adsorbed to ribosomes, but specifically located towards the 5′‐end of the mRNA chains in polyribosomes.