HIV-1 reverse transcriptase connection subdomain mutations reduce template RNA degradation and enhance AZT excision.

We previously proposed that mutations in the connection subdomain (cn) of HIV-1 reverse transcriptase increase AZT resistance by altering the balance between nucleotide excision and template RNA degradation. To test the predictions of this model, we analyzed the effects of previously identified cn mutations in combination with thymidine analog mutations (D67N, K70R, T215Y, and K219Q) on in vitro RNase H activity and AZT monophosphate (AZTMP) excision. We found that cn mutations G335C/D, N348I, A360I/V, V365I, and A376S decreased primary and secondary RNase H cleavages. The patient-derived cns increased ATP- and PPi-mediated AZTMP excision on an RNA template compared with a DNA template. One of 5 cns caused an increase in ATP-mediated AZTMP excision on a DNA template, whereas three cns showed a higher ratio of ATP- to PPi-mediated excision, indicating that some cn mutations also affect excision on a DNA substrate. Overall, the results strongly support the model that cn mutations increase AZT resistance by reducing template RNA degradation, thereby providing additional time for RT to excise AZTMP.