Characterization of a ribonuclease from Anacystis nidulans infected with cyanophage as-1

In Anacystis nidulans the ribonuclease (RNase) activity is very low but is greatly increased upon phage-infection. A RNase was isolated and purified over 300-fold from A. nidulans cells infected by cyanophage AS-1. The enzyme did not attack single- or double-stranded DNA, was inactive on p-nitrophenyl phosphate or bis-p-nitrophenyl phosphate as substrates, and had neither 3′- nor 5′-nucleotidase activity. The approximate MW of the enzyme was 12000. Maximal enzyme activity was at pH 7.5. No absolute requirement for metal ions was observed, but Fe³⁺ stimulated and Co²⁺ and Ni²⁺ inhibited enzyme activity. The enzyme is an endonuclease which, upon exhaustive hydrolysis, produces mainly oligonucleotides (average chain-length: 3) with 3′-P termini. Analysis of the base composition of these oligonucleotides and determination of their 3′-terminal nucleosides, together with the investigation of the rate of hydrolysis of synthetic polyribonucleotides, have shown that the enzyme has a relative specificity for uridylic acid.