Chlorella virus SC-1A encodes at least five functional and one nonfunctional DNA methyltransferases

Chlorella virus SC-1A encodes at least six DNA methyltransferases (MTases): four N⁶-methyldeoxyadenine (m⁶A) MTases, M·CviSI (TGC^mA), M·CviSII (C^mATG), M·CviSIII (TCG^mA) and M·CviSIV (G^mATC), one 5-methyldeoxycytosine (m⁵C) MTase, M·CviSV (~ RC^mCG), and one nonfunctional m⁵C MTase, M·CviSVI, which is homologous to the MTase M·CviJI [RG^mC(T/C/G)] produced by another chlorella virus IL-3A. Genes encoding three of the SC-1A m⁶A MTases (M·CviSI, M·CviSII, and M·CviSIII) and the nonfunctional m⁵C MTase were cloned and sequenced. Neither M·CviSI nor M·CviSIII genes hybridized to genes for their respective isomethylomers, M·CviRI and M·CviBIII, from other chlorella viruses. However, the M·CviSII gene hybridized strongly to its M·CviAII isomethylomer gene from virus PBCV-1. Like the prototype chlorella virus PBCV-1, the SC-1A genome contains inverted terminal repeats, one of which is adjacent to the nonfunctional m⁵C MTase. The three cloned m⁶A MTase genes are distributed throughout the approx. 345 kb SC-1A genome.