The selective degradation of phage-induced ribonucleic acid by polynucleotide phosphorylase.

In bacteria infected by T-even bacteriophages, the net synthesis of ribonucleic acid stops immediately after infection although the synthesis of protein continues actively (1, 2). However, with the use of an isotope-labeling technique, Volkin and Astrachan (3) demonstrated that a minor fraction of the total ribonucleic acid is produced soon after phage infection and that the apparent nucleotide composition of this ribonucleic acid corresponds to that of the deoxyribonucleic acid of the infecting phages. The synthesis of this phage-induced RNA may be correlated with the early synthesis of phage-induced enzymes, such as the deoxycytidylate hydroxymethylase (4). It has also been found that the bulk of phage-induced RNA is associated with the ribosomes of the infected cells (5-8). These findings are a major basis of a hypothesis that a special class of RNA molecules, called “messenger” RNA, whichin this instance has been equated with phage-induced RNA, is formed on the template of DNA and carries the information from DNA to ribosomes where protein is synthesized. Turnover experiments show that the phage-induced RNA is degraded quickly after its formation and that some of its labeled constituents are incorporated into the phage DNA which is formed subsequently (9). Since this instability is listed as one of the characteristic properties of “messenger” RNA, it seems of interest to study what enzymatic reactions are involved in this process. In a previous paper from this laboratory (lo), it was shown that the phage-induced RNA is efficiently converted to deoxyribonucleotides in extracts of phage-infected cells. This over-all conversion was separated into at least two different processes, i.e. the selective degradation of the phage-induced RNA to 5’-ribonucleotides and the reduction of ribonucleotides to deoxyribonucleotides. In this paper we report on studies on the selective degradation of phage-induced RNA, with the use of a more completely resolved system than that used in the earlier study. In these experiments a requirement for inorganic orthophosphate in the selective degradation of phage-induced RNA in ribosomes has been established. An analysis of the degradation products and experiments with appropriate inhibitors indicate that polynucleotide phosphorylase has an essential role in the degradation of the phage-induced RNA. A phosphodiesterase has also been implicated in the degradation. The action of poly-