Hundreds of large polyhedral, plaque-forming viruses that infect Chlorella NC64A have been isolated from the USA, South America, China, Japan, Israel and Australia. Over 50 of these viruses, called NC64A viruses, have been partially characterized. Like PBCV-1, each of these viruses contains many structural proteins, a large (>300 kb) dsDNA genome with about a 40% GC content, and they are chloroform sensitive. The DNAs of some of these viruses hybridize strongly with PBCV-1 DNA, while others hybridize poorly.
The NC64A viruses can be grouped into at least 16 classes by plaque size, antiserum sensitivity, DNA restriction patterns, sensitivity of the DNAs to restriction endonucleases and the nature and abundance of methylated bases. DNA from each of the viruses contains 5-methylcytosine (5mC) in amounts varying from 0.12 to 47.5% of the total cytosine (Table 1). In addition, many viral DNAs contain N6-methyladenine (6mA) in amounts varying from 1.5 to 37% of the total adenine.
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