Identification of a nuclease and host restriction-modification in the unicellular, aerobic nitrogen-fixing cyanobacterium Cyanothece sp.

In the process of developing a gene transfer system for the marine, unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. strain BH68K, two major restriction barriers have been identified. A cell wall-associated nuclease exhibited non-site-specific degradation of covalently closed circular and linear double-stranded DNA molecules, including Cyanothece sp. strain BH68K chromosomal DNA. The nuclease is easily released from intact cells by using water or buffer containing Triton X-100. Nuclease activity was undetectable in cell extracts prepared from water-washed cells. Comparison of the restriction endonuclease susceptibility of Cyanothece sp. strain BH68K DNA to that of Anabaena sp. strain PCC 7120 revealed that these organisms have a nearly identical pattern of restriction and therefore may contain similar systems for DNA methylation. Restriction by DpnI, MboI, and Sau3AI indicated the presence of adenine methylation. Cyanothece sp. strain BH68K cell extracts contain a type II restriction endonuclease, Csp68KI. The activity of Csp68KI was easily detected in cell extracts without extensive purification. Csp68KI is an isoschizomer of AvaII and recognizes the nucleotide sequence 5'-GG(A/T)CC-3'. Cleavage occurs between the guanosine nucleotides producing 3-bp 5' overhang ends.