The chlorelia virus PBCV-1 contains an open reading frame, named P17-ORF4, which differs by eight amino acids from a DNA cytosine methyltransferase, M.CviJI, encoded by a different chlorelia virus IL-3A. Whereas IL-3A expresses M.CviJI, which methylates the central cytosine in (A/G)GCT/C/G) sequences, P17-ORF4 is non-functional. Gene fusions between P17-ORF4 and M.CvUI and site-directed point mutations revealed that changing Gin188 to Lys188 abolishes M.CviJI methyltransferase activity. Conversely, changing Lys188 in P17-ORF4 to Gin188 results in M.CvUI activity. The other altered seven amino acids do not appear to affect M.CviJI activity.
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