Methylation and expression of bovine leukemia proviral DNA.

In vivo, the BLV proviral DNA usually resides in a transcriptionally inert state, and is hypermethylated. Upon short-term in vitro cultivation of the neoplastic or non-neoplastic lymphoid cells, the viral genome becomes transcriptionally active but without detectable change in its methylation state. Proviral DNA was found to be methylated in one but not in the other long-term BLV producer cell line examined. These data indicate that hypermethylation of proviral DNA may not be responsible for the covert nature of BLV infection in vivo.