Simultaneous ternary extension of DNA catalyzed by a trimeric replicase assembled in vivo.

According to current models, dimeric DNA Polymerases coordinate the replication of DNA leading and lagging strands. However, it was recently shown that trimeric DNA Polymerases, assembled in vitro, replicate the lagging strand more efficiently than dimeric replicases. Here we show that the t, a, ε, and q subunits of Escherichia coli DNA Polymerase III can be assembled in vivo, yielding the trimeric t3a3ε3q3 complex. Further, we propose a molecular model of this complex, whose catalytic action was investigated using model DNA substrates. Our observations indicate that trimeric DNA replicases reduce the gap between leading and lagging strand synthesis.