In the ternary substrate complex of DNA polymerase (pol) beta, the nascent base pair (templating and incoming nucleotides) is sandwiched between the duplex DNA terminus and polymerase. To probe molecular interactions in the dNTP-binding pocket, we analyzed the kinetic behavior of wild-type pol beta on modified DNA substrates that alter the structure of the DNA terminus and represent mutagenic intermediates. The DNA substrates were modified to 1) alter the sequence of the duplex terminus (matched and mismatched), 2) introduce abasic sites near the nascent base pair, and 3) insert extra bases in the primer or template strands to mimic frameshift intermediates. The results indicate that the nucleotide insertion efficiency (k(cat)/K(m), dGTP-dC) is highly dependent on the sequence identity of the matched (i.e. Watson-Crick base pair) DNA terminus (template/primer, G/C approximately A/T > T/A approximately C/G). Mismatches at the primer terminus strongly diminish correct nucleotide insertion efficiency but do not affect DNA binding affinity. Transition intermediates are generally extended more easily than transversions. Most mismatched primer termini decrease the rate of insertion and binding affinity of the incoming nucleotide. In contrast, the loss of catalytic efficiency with homopurine mismatches at the duplex DNA terminus is entirely due to the inability to insert the incoming nucleotide, since K(d)((dGTP)) is not affected. Abasic sites and extra nucleotides in and around the duplex terminus decrease catalytic efficiency and are more detrimental to the nascent base pair binding pocket when situated in the primer strand than the equivalent position in the template strand.