Reversion of a Moloney murine leukemia virus RNase H mutant at a second site restores enzyme function and infectivity

The reverse transcriptase of retroviruses contains an RNase H activity essential for the proper synthesis of the viral DNA copy of the RNA genome. We have previously characterized a number of point mutations altering the RNase domain of the Moloney murine leukemia virus reverse transcriptase (S. W. Blain and S. P. Goff, J. Biol. Chem. 268:23585-23592, 1993). One such mutation, Y586F (a Y-to-F change at position 586), reduced RNase H activity, as assayed by in situ gel analysis, to about 5% of the wild-type level and prevented viral replication. We have now recovered a revertant virus with near-normal infectivity and in vitro enzymatic activity. The revertant contains a single substitution, N613H, distant in the primary sequence of the protein, but modeling with the Escherichia coli RNase H structure suggests that the reverted residue is close in space to the original substituted residue. Examination of the structure permits some suggestions as to how this second-site revertant restores enzyme activity.