Evidence for a Restriction/Modification‐Like System in Anacystis nidulans Infected by Cyanophage AS‐1

Anacystis nidulans infected by AS‐1 cyanophage contains an endnuclease (AS‐1 endonuclease) which splits host DNA but not AS‐1 phage DNA [Szekeres, M. (1981) Virology, III, 1–10]. AS‐1 phage DNA proved to be resistant not only to AS‐1 endonuclease but also to a number of restriction endonucleases the recognition sites of which contain a central dG‐dC dinucleotide. Since an unmodified 5′dG‐dC dinucleotide was shown to be present at the sites at which DNA is cleaved by AS‐1 endonuclease, the results suggest that the sites attacked preferentially by the AS‐1 endonuclease are specifically protected on the AS‐1 DNA molecule. The modification of AS‐1 DNA was shown to occur specifically in infected Anacystis because AS‐1 DNA fragments which are normally resistant to AS‐1 endonclease became susceptible to this enzyme if inserted into pBR322 plasmid and cloned in Escherichia coli. AS‐1 DNA was shown to contain about 5% of a modified nucleotide which was not 5‐methyldeoxycytidylic acid. Results presented and our earlier data suggest that in Anacystis infected by AS‐1 phage, a restriction/modification‐like system operates which is able to eliminate ‘unwanted’ (host) DNA selectively.