Investigation of sequence homology in a group of type-II restriction/modification isoschizomers

We have dissected the cloned PstI M and R genes to make DNA hybridization probes spanning most of the sequence. These subclones, and also the intact sequence, were used to search for nucleic acid homology by Southern blot in the DNA from twelve organisms which produce PstI isoschizomers. One of these probes, a 206-bp fragment from the N-terminal domain of the endonuclease, showed significant hybridisation in four strains (Escherichia coli strains RFL48, RFL49 and RFL83, and Streptomyces albus P). No significant hybridisation was detected with other parts of the PstI sequences. We have used computer similarity searches to look for homology between the PstI proteins and the known sequences of other type-II systems that recognise different sites. We postulate a possible recognition domain within the M.PstI methyltransferase based on similarity to the M.PaeR7 and M.TaqI methyltransferases.