Mesophilic cyanobacteria producing thermophilic restriction endonucleases

When searching for the site-specific endonucleases in several strains of Ph ormidium we made the following observations. Among the 16 strains that orig inated from 15 species of Phormidium. 12 produced one or more restriction e nzymes, of which two produced the highly thermophilic restriction endonucle ases PtaI and PpaAII with their optimum activity at 65-80 degreesC, which i s far above the lethal temperature for the host microorganism (40 degreesC) . These two temperature-resistant enzymes are isoschizomers of known BspMII and TaqI endonucleases, respectively. The presence of the thermophilic Taq I isoschizomer does not seem to play any role in the mesophilic host microo rganism, which does not even contain an active cognate methyltransferase. A mong the remaining 10 strains, six produced isoschizomers of endonucleases which we first described in cyanobacteria, namely:PfuAII (NdeI), PinBII and PtaI(BspMII). PlaAII (RsaI), PpaAII PpeI (ApaI). Two enzymes, PauAII (AhaI II) and PfaAII (NdeI), belong to a group of a very rarely occurring isoschi zomers. Out oi 21 cyanobacterial endonucleases investigated by us, Four wer e active in a wide range of temperatures (from 15 to 60 degreesC) which als o extended the optimal growth temperature of the hosts. We assume that our observation on the presence of temperature-resistant restriction enzymes in mesophilic hosts supports the idea of horizontal gene transfer. Restrictio n modification systems may be an excellent tool for investigation of that p henomenon.(C) 2001 Federation of European Microbiological Societies. Publis hed by Elsevier Science B.V. All rights reserved.