The molecular basis of nucleotide recognition for T7 DNA polymerase
DNA replication demands extraordinary specificity and efficiency of catalysis from a DNA polymerase. Previous studies on several DNA polymerases suggested that a rate-limiting conformational change preceding chemistry accounts for the high specificity following the induced fit mechanism. However, the identity of this rate-limiting conformational change and how it contributes to the fidelity is still under debate. An important study of T7 DNA polymerase performed by Tsai and Johnson using a conformationally sensitive fluorophore (CSF) characterized a conformational change directly and presented a new paradigm for nucleotide selectivity. This thesis describes work to further characterize the underlying molecular basis regulating the conformational change by a combination of site-directed mutagenesis, transient kinetics and crystallography. One flexible segment (gly-ala-gly) within the fingers domain was mutated to (ala-alaala). The kinetic analysis on this mutant showed that the mutations decreased the forward rate of the conformational change reported by the fluorophore about 1200-fold but there was no significant change on the reverse rate. The data suggested that the movement of the fingers domain is not a rigid body motion but may be complex due to the movements of various helices within the fingers domain. Quantification of the kinetics of incorporation of correct and incorrect base pairs showed the decrease of fidelity mainly was from the decreased forward rate during correct nucleotide incorporation. The roles of three active site residues, K522, H506, and R518, which form polar interactions with [alpha]-,[beat]- and [gamma]-phosphates of the incoming nucleotide respectively, in conformational change and catalysis were also characterized. All the mutants showed a slower conformational change than the wild type enzyme. After this conformational change, there was a rate limiting step with a rate comparable to kpol measured by quench-flow experiments. Correct nucleotide binding caused an increase in fluorescence, suggesting that the conformational change of the fingers domain delivers incoming nucleotide to a misaligned status even for a correct nucleotide with each of the mutants. The data suggested that active site residues play important roles in maintaining a fast conformational change and an accurate alignment of the active site during correct nucleotide incorporation. Yellow crystals of CSF-labeled T7 DNA polymerase with DNA and correct nucleotide (closed complex), incorrect nucleotide (misaligned complex) or no nucleotide (open complex) were grown to good size and diffracted to 3 Å during X-ray data collection. The structures of these complexes are still under refinement.