CHLORELLA VIRUS SC-1A ENCODES AT LEAST 5 FUNCTIONAL AND ONE NONFUNCTIONAL DNA METHYLTRANSFERASES

Chlorella virus SC-IA encodes at least six DNA methyltransferases (MTa ses): four N-6-methyldeoxyadenine (m(6)A) MTases, M.CViSI(TGC(m)A), M. CviSII (C(m)ATG), M.CviSIII (TCG(m)A) and M.CviSIV (G(m)ATC), one 5-me thyldeoxycytosine (m(5)C) MTase, M.CviSV (similar to RC(m)CG), and one nonfunctional m(5)C MTase, M.CviSVI, which is homologous to the MTase M.CviJI [RG(m)C(T/C/G)] produced by another chlorella virus IL-3A. Ge nes encoding three of the SC-1A m(6)A MTases (M.CviSI, M.CviSII, and M .CviSIII) and the nonfunctional m(5)C MTase were cloned and sequenced. Neither M.CviSI nor M.CviSIII genes hybridized to genes for their res pective isomethylomers, M.CviRI and M.CviBIII, from other chlorella vi ruses. However, the M.CviSII gene hybridized strongly to its M.CviAII isomethylomer gene from virus PBCV-1. Like the prototype chlorella vir us PBCV-1. the SC-1A genome contains inverted terminal repeats, one of which is adjacent to the nonfunctional m(5)C MTase. The three cloned m(6)A MTase genes are distributed throughout the approx. 345 kb SC-1A genome. (C) 1997 Elsevier Science B.V.