Isolation and genetic structure of theAvallisoschizomeric restriction-modification systemHgiBI fromHerpetosiphon giganteus Hpg5: M.HgiBl reveals high homology to M.Banl

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Abstract
The complete type II restriction-modification system HgiBl of Herpetosiphon giganteus strain Hpg5 recognizing the Avail specific DNA sequence GGWCC has been cloned and expressed functionally active in Escherichla coli. A considerable acceleration in cloning could be acchieved 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.HgiBl to the group of m5 methyltransferases, especially to M.BanI from Bacillus aneurlnolyticus with the related recognition sequence GGYRCC. In contrast, no significant similarities have been observed for the associated endonuclease R.HgiBl with any other restriction enzyme described so far, even not with the isoschlzomeric R.Sinl from Salmonella infantis, or with R.BanI.