Characterization of a terminal deoxynucleotidyl transferase activity in mouse mammary tumor virus

Mouse mammary tumor virus (MMTV) contains an enzyme which catalyzes DNA synthesis in the presence of a preformed initiator without template direction. This type of enzyme activity is characteristic of the class of polymerases called terminal deoxynucleotidyl transferase. The MMTV enzyme is localized in the viral core and its activity depends upon the presence of detergent, reducing agent, preformed initiator, and divalent cation. Optimal conditions for activity include pH 6.85 and the use of a thymidine-containing substrate and initiator. The reaction product is predominantly single stranded. Concentration-dependent inhibition by several salts is observed. Manipulation of the reaction optima and salt concentration allows the functional separation of MMTV terminal transferase activity from RNA-dependent DNA polymerase activity.