Role of Q190 of MuLV RT in ddNTP resistance and fidelity of DNA synthesis:a molecular model of interactions with substrates

Gln190 of MuLV reverse transcriptase (RT) plays an important role in the ca talytic mechanism of MuLV RT for its conservative and non-conservative muta nt derivatives exhibit low catalytic activity. We now report that both Q190 N and Q190A MuLV RTs are more efficient in their activity to incorporate dd NTPs and exhibit higher fidelity than the wild-type (WT) enzyme of DNA synt hesis in both RNA- and DNA-directed reactions. To obtain some insight into the structural basis for the differential utilization of dNTP and ddNTP by the mutant enzymes, we modeled the binary and the ternary complexes of MuLV RT using corresponding HIV-1 RT structures and available structure of the fragment of MuLV RT. Q190 of MuLV RT appears to be essential for the intera ction with 3'OH of dNTP. The lack of a 3'OH moiety in ddNTP does not permit the binding of ddNTPs to WT MuLV RT. However, the shorter side chain of Q1 90N (or A) mutant MuLV RT and the absence of 3'OH in ddNTP result in the re arrangement of hydrophobic interactions favoring the binding and limited in corporation of ddNTPs. In addition, while modeling the binary and ternary c omplexes of MuLV RT, we noted that in the formation of the ternary complex, an interaction of Q190 with dNTP substrate requires a shift from its inter action with the template base. This may be achieved by a small conformation al change or motion of the loop between betas and alphaH containing Q190, w hich may correspond to the conformational change step requiring participati on of Q190 during the catalytic reaction as reported in an earlier biochemi cal investigation.