| Abstract: | In toluene-treated cells of Bacillus brevis, newly synthesized RNA is rapidly degraded in a reaction that is inhibited by cyclic guanosine 3':5'-monophosphate (cGMP) and by 1,10-phenanthroline. This appears to be due to a ribonuclease found in cell-free extracts of B. brevis which is inhibited by cGMP and related compounds as well as by 1,10-phenanthroline. The cGMP-sensitive nuclease hydrolyzes synthetic polynucleotides, yielding nucleoside 5'-monophosphates as the sole products, even during the early stages of hydrolysis. Synthetic polynucleotides terminated by a 3'-phosphate are resistant to hydrolysis. While with 3'-hydrolysis of the polymer. The enzyme is therefore an exonuclease that degrades polynucleotides from the 3' end to product 5'-mononucleotides. It also acts on denatured but not on native DNA. Activity is greatest in the presence of Mn2+ and is not affected by the presence of monovalent cations. 1,10-Phenanthroline, but not 1,7-phenanthroline, inhibits the nuclease even when Mn2+ is present in excess. The inhibition of the enzyme by cGMP is noncompetitive, and cGMP itself is not hydrolyzed. The sensitivity of the nuclease to inhibition depends strikingly on the nature of the substrate and is lost when the enzyme is assayed at high pH. These observations suggest that cGMP inhibits the nuclease by combining with an allosteric site on the enzyme. Although cGMP was found to be the most effective inhibitor, other nucleoside 3':5'-monophosphates and derivatives of 5'-GMP can also inhibit the nuclease. Since measurements of cGMP in B. brevis have not revealed detectable amounts (less than 5 times 10-8 M), the substance that modulates the activity of the nuclease under physiological conditions remains to be identified. |
