Characterization of an HIV-1 Isolate Displaying an Apparent Absence of Virion-Associated Reverse Transcriptase Activity

In characterizing a group of independent human immunodeficiency virus (HIV-1) isolates, we noted that certain isolates had anomolously low levels of virion-associated reverse transcriptase activity. In an attempt to understand the basis of this phenomenon, we examined in detail one such isolate, HIV-1_G. We found correctly processed forms of the viral reverse transcriptase in virions as well as processed forms of other viral proteins, suggesting that viral proteins are both expressed and properly processed. We have detected a nuclease activity associated with the outer face of the HIV-1_G envelope. This nuclease degrades the DNA product generated during the reverse transcription assay. The nuclease activity is more sensitive to mild protein denaturation than is the viral reverse transcriptase, and it is stimulated by the presence of Ca²⁺. The amount of virion-associated nuclease activity relative to reverse transcriptase activity varies between virus isolates and can vary also for one isolate during virus spread through a culture. The origin of the nuclease activity is unknown but is presumed to be cellular. The variability in amount of nuclease activity may reflect variability in the interaction of the virus with different cellular components during maturation.