Helix clamp motifs of polymerases possessing the helix-turn-helix secondary Structure are Crucial for their polymerase activity by binding to the nucleic acid template/primer via the alpha helices. To study the functions of turn in helix clamp motif of human immunodeficiency virus (HIV)-1 RT, clones with turn mutants at rt271-274 of HIV-1 RT were generated and Studied by one cycle infection assay. Mutants rtY271A and rt1274A almost lost their replication competency, while mutants rtA272P, rtA272S, and rtG273A retained comparable replication competency relative to wild type pseudotyped HIV-1. To Study the mechanisms involved, RT proteins from rt271 to rt274 mutants were expressed and assayed for their RNA dependent DNA polymerase activity, DNA binding activity and processivity. Discordance between RT activity and viral replication efficiency of some turn mutants was observed, indicating that aside from RT, other steps in HIV replication Could be affected by substitutions at the turn of helix clamp motif. (C) 2008 Elsevier Inc. All rights reserved.