Phage-induced development of a site-specific endonuclease in Anacystis nidulans, a cyanobacterium

Cleavage and sequence recognition of 2,6-diaminopurine-containing DNA by site-specific endonucleases

The susceptibility of 2,6-diaminopurine (DAP)-containing bacteriophage DNA to several restriction and other endonucleases was examined. With the only exception of TaqI, these enzymes did not accept the modified base as a substitute for adenine. The phage DNA was extensively fragmented by the restriction endonucleases which recognize only G and C-containing sites. 5'-terminal analysis of the MspI and SmaI fragments revealed that d(DAP-T) basepairs can be mistaken by some enzymes for d(G-C) pairs.

Evidence for a restriction/modification-like system in Anacystis nidulans infected by cyanophage AS-1

Anacystis nidulans infected by AS-1 cyanophage contains an endonuclease (AS-1 endonuclease) which splits host DNA but not AS-1 phage DNA [Szekeres, M. (1981) Virology, 111, 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 endonuclease 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.