Alanine-scanning mutations in the "primer grip" of p66 HIV-1 reverse transcriptase result in selective loss of RNA priming activity.
Alanine-scanning mutants of the primer grip region of human immunodeficiency virus type 1 reverse transcriptase were tested for their ability to extend RNA and DNA versions of the polypurine tract primer, and an oligonucleotide representing the 18-nucleotide sequence at the 3' end of tRNALys3. A majority of the mutant enzymes were either completely or severely deficient in RNA priming activity, but, with only one exception, were able to efficiently extend DNA versions of the same primers. The mutant enzymes were able to bind to RNA primers, indicating that the defect in RNA priming was not simply a loss of binding activity. Mutations at positions 229, 233, and 235 dramatically reduced the amount of specific RNase H cleavage at the 3' terminus of the polypurine tract, which is required for primer removal. An alanine substitution at position 232 led to loss of cleavage specificity, although total activity was close to the wild-type level. Taken together, these results demonstrate for the first time that there are residues in human immunodeficiency virus type 1 reverse transcriptase which are specifically involved in protein-nucleic acid interactions with RNA primers.