Excretion of 5′-Nucleotides by Bacteria

A Bacillus accumulated 5′-nucleotides in the culture fluid during cultivation. These nucleotides consisted mainly of ribonucleoside 5′-monophosphates and of traces of nucleoside 5′-diphosphates. The composition of the accumulated nucleotides resembled that of the nucleotides of intracellular RNA. Therefore an intimate relationship between the accumulation of nucleotides and the metabolism of endogenous RNA was suggested. The excretion of 5′-nucleotides into the culture fluid took place in parallel with the reduction of intracellular RNA, and it was presumed that the accumulated nucleotides were products of degradation of intracellular RNA. For the purpose of clarifying the mechanism of extracellular formation of 5′-nucleotides during cultivation of a Bacillus, which had been reported previously1), attempts were made to form 5′-nucleotides by non-proliferating cells of this Bacillus. It was found that this Bacillus rapidly degrades endogenous RNA and consequently forms 5′-nucleotides extracellularly, when the washed cells are incubated in alkaline salt or buffer solutions. Comparison of the process of the excretion of 5′-nucleotides from washed cells with that of the accumulation of 5′-nucleotides during cultivation revealed that both were brought about by the same mechanism, i.e., the degradation of endogenous RNA. Various factors which affect the degradation of endogenous RNA were also examined and the biological significance of this kind of RNA-degradation was discussed. The degradation of endogenous RNA in a Bacillus was studied as a continuation of the preceding reports1,2). The enzyme participating in this degradation was strongly suggested to be polynucleotide phosphorylase from the findings that Mg++ and orthophosphate were required for the degradation, orthophosphate was incorporated in parallel with the degradation of RNA and ribonucleoside 5′-diphosphates were formed as breakdown products. The presence of polynucleotide phosphorylase was also confirmed by synthesizing poly-nucleotides from ribonucleoside 5′-diphosphates with an enzyme preparation from this organism. Part of this enzyme precipitated together with ribonucleoprotein particles by a 105,000×g centrifligation while the major proportion remained in the supernatant fluid. The biological role of this enzyme was also discussed.