LOWER IN-VIVO MUTATION-RATE OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 THAN THAT PREDICTED FROM THE FIDELITY OF PURIFIED REVERSE-TRANSCRIPTASE

level of genetic variation of human immunodeficiency virus type 1 (HIV -1), a member of the lentivirus genus of the Retroviridae family, is h igh relative to that of retroviruses in some other genera, The high er ror rates of purified HIV-1 reverse transcriptase in cell-free systems suggest an explanation for this high genetic variation, To test wheth er the in vivo rate of mutation during reverse transcription of HIV-1 is as high as predicted by cell-free studies, and therefore higher tha n that rates of mutation of retroviruses in other genera, we developed an in vivo assay for detecting forward mutations in HIV-1, using the lacZ alpha peptide gene as a reporter for mutations, This system allow s the rates and types of mutations that occur during a single cycle of replication to be studied, We found that the forward mutation rate fo r HIV-1 was 3.4 x 10(-5) mutations per bp per cycle, Base substitution mutations predominated; G-to-A transition mutations were the most com mon base substitution, The in vivo mutation rates for HIV-1 are three and seven times higher than those previously reported for two other re troviruses, spleen necrosis virus and bovine leukemia virus, respectiv ely, In contrast, our calculated in vivo mutation rate for HIV-1 is ab out 20-fold lower than the error rate of purified HIV-1 reverse transc riptase, with the same target sequence, This finding indicates that HI V-1 reverse transcription in vivo is not as error prone as predicted f rom the fidelity of purified reverse transcriptase in cell-free studie s, Our data suggest that the fidelity of purified HIV-1 reverse transc riptase may not accurately reflect the level of genetic variation in a natural infection.