Restriction barrier composed of an extracellular nuclease and restriction endonuclease in the unicellular cyanobacterium Microcystis sp.

The unicellular cyanobacterium Microcystis aeruginosa K-81 has two types of restriction barrier, an extracellular nuclease and sequence-specific endonucleases. The nuclease was detected in the culture supernatant and it was easily released from the cells by washing with water or buffer containing Triton X-100. This nuclease was identified as a polypeptide of about 28 kDa that digested covalently closed circular and linear double-stranded DNAs, including chromosomal DNA from M. aeruginosa K-81. Among another 13 Microcystis strains examined, 3 produced an extracellular nuclease. Furthermore, M. aeruginosa K-81 contained two sequence-specific endonucleases, MaeK81I and MaeK81II, which were isoschizomers of SplI and Sau96I, respectively.