Isolation and genetic structure of the AvaII isoschizomeric restriction-modification system HgiBI from Herpetosiphon giganteus Hpg5: M.HgiBI reveals high homology to M.BanI.

The complete type II restriction-modification system HgiBI of Herpetosiphon giganteus strain Hpg5 recognizing the AvaII specific DNA sequence GGWCC has been cloned and expressed functionally active in Escherichia coli. A considerable acceleration in cloning could be achieved by preparing a size restricted library after application of a related hybridization probe. Both methyltransferase (437 codons) and restriction endonuclease gene (274 codons) were found to be encoded on a 3.6 kilobases ClaI/HincII fragment in the same transcriptional orientation separated by one triplett only. Protein sequence comparisons revealed a close resemblance of M.HgiBI to the group of m5C-methyltransferases, especially to M.BanI from Bacillus aneurinolyticus with the related recognition sequence GGYRCC. In contrast, no significant similarities have been observed for the associated endonuclease R.HgiBI with any other restriction enzyme described so far, even not with the isoschizomeric R.SinI from Salmonella infantis, or with R.BanI.