DSAV METHYLTRANSFERASE AND ITS ISOSCHIZOMERS CONTAIN A CONSERVED SEGMENT THAT IS SIMILAR TO THE SEGMENT IN HHAI METHYLTRANSFERASE THAT IS IN CONTACT WITH DNA BASES

The methyltransferase (MTase) in the DsaV restriction - modification s ystem methylates within 5'-CCNGG sequences. We have cloned the gene fo r this MTase and determined its sequence. The predicted sequence of th e MTase protein contains sequence motifs conserved among all cytosine- 5 MTases and is most similar to other MTases that methylate CCNGG sequ ences, namely M.ScrFl and M.Ssoll. All three MTases methylate the inte rnal cytosine within their recognition sequence. The 'variable' region within the three enzymes that methylate CCNGG can be aligned with the sequences of two enzymes that methylate CCWGG sequences. Remarkably, two segments within this region contain significant similarity with th e region of M.Hhal that is known to contact DNA bases. These alignment s suggest that many cytosine-5 MTases are likely to interact with DNA using a similar structural framework.