DNA polymerase I: structure, activity, and function in bacterial DNA replication and repair

Faithful replication and repair of the genome are essential processes for all life. Genome maintenance is coordinated by a complex suite of proteins, with bacteria evolving intricate systems despite their relatively simplistic genomes. DNA polymerases are a key class of proteins that mediate genome maintenance. DNA polymerases are all capable of extending nascent strands of DNA but contribute to DNA replication and repair in distinct ways depending on their active site and substrate specificity. The first discovered polymerase, bacterial DNA polymerase I (Pol I), has long been considered the primary enzyme responsible for Okazaki fragment maturation and resynthesis in many DNA repair pathways. These conclusions derive primarily from studies using the gram-negative bacterium, Escherichia coli. Given that some bacterial lineages diverged from E. coli over a billion years ago, these assumptions may not account for evolution in functional diversity. In this review, we examine the structural features of bacterial Pol I and discuss how each of its distinct enzymatic activities contribute to genome maintenance. Throughout, we introduce differences that have been discovered between gram-negative and gram-positive species and explore how activity differences may translate to functional adaptations in replication or repair. We focus on evidence from gram-positive bacteria, particularly Bacillus subtilis and Geobacillus stearothermophilus, that challenges the universality of Pol I's functions and reveals lineage-specific adaptations in replication and repair mechanisms. By synthesizing historical perspectives with recent discoveries, this review underscores both the importance of Pol I and the evolutionary diversification of Pol I in bacterial DNA metabolism.