Chlorella viruses encode multiple DNA methyltransferases

The >320 kb dsDNA genomes of 16 viruses which infect Chlorella strain NC64A and 5 viruses infecting Chlorella strain Pbi were tested for their sensitivity/resistance to more than 80 DNA restriction endonucleases. From the known methylation sensitivities of these enzymes to site-specific 5-methylcytosine and N6-methyladenine DNA modifications, we deduce that the 16 NC64A viruses encode at least 13 different sequence-specific DNA methyltransferases and the 5 Pbi viruses encode at least 7 sequence-specific DNA methyltransferases. Each DNA methyltransferase has a 2 to 4 base pair DNA recognition sequence. Some individual viruses encode as many as ten different DNA methyltransferases, making these chlorella virus genomes among the most concentrated sources of DNA methyltransferase genes known.