STUDIES ON THE FORMATION OF TOBACCO MOSAIC VIRUS RIBONUCLEIC ACID, VI. MODE OF DEGRADATION OF HOST RIBONUCLEIC ACID TO RIBONUCLEOSIDES AND THEIR CONVERSION TO RIBONUCLEOSIDE 5′-PHOSPHATES

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Abstract
Partial purification and properties of tobacco leaf enzymes, involved in the degradation of cellular RNA to ribonucleosides and their conversion to ribonucleoside 5''-phosphates, have been described. Ribonuclease (TLRNAase-II) associated with the microsomes has a pH optimum of 5.8-6.0 in citrate-phosphate buffer. The degradation of RNA proceeds via transphosphorylation followed by hydrolysis of both the purine and pyrimidine 2'',3''-cyclic nucleotides to their corresponding nucleoside 3''-phosphates. The action of TLRNAase-II differs from that of the ribonuclease (TLRNAase-I), isolated earlier from tobacco leaves, which hydrolyzes only purine 2'',3''-cyclic nucleotides to their corresponding nucleoside 3''-phosphates. TLRNAase-II is loosely bound to the microsomes. Phosphatase, associated with the microsomes, is a nonspecific type and is similar to the one isolated earlier from tobacco leaves. It hydrolyzes ribonucleoside 3''-phosphates, formed as a result of the degradation of RNA, to ribonucleosides. Nucleoside phosphokinase, associated with the microsomes, has a pH optimum of 5.6 in sodium acetate buffer. It catalyzes the formation of ribonucleoside 5''-phosphates by the transfer of phosphate from ATP or ADP or AMP to ribonucleosides. Mode of degradation of cellular RNA to ribonucleosides and their conversion to ribonucleoside 5''-phosphates is summarized below: [image] These reactions provide a pathway for the biosynthesis of TMV-RNA in which ribonucleosides derived from host RNA can serve as intermediates.