5-Azacytidine-resistant mutants of Chlorella virus IL-3A

Many dsDNA-containing viruses which infect the unicellular, eukaryotic Chlorella-like green alga strain NC64A encode for DNA methyltransferases and DNA site-specific (restriction) endonucleases. We have hypothesized that these endonucleases help degrade host DNA permitting deoxynucleotides to recycle into virus DNA. This hypothesis was tested by isolating deletion mutants of Chlorella virus IL-3A lacking functional genes for the cytosine methyltransferase M.CviJI and the cognate site-specific endonuclease CviJI. The growth and burst sizes of the mutants and parent virus were identical. Also host nuclear and chloroplast DNAs disappeared from infected cells at the same rates. Thus M.CviJI and CviJI activities are not required for IL-3A replication and CviJI activity is not essential for host DNA degradation.