Analysis of the distribution and evolution of the ATP-dependent DNA ligases of bacteria delineates a distinct phylogenetic group 'Lig E'.

Prior to the discovery of a minimal ATP-dependent DNA ligase in Haemophilus influenzae, bacteria were thought to only possess a NAD-dependent ligase, which was involved in sealing of Okazaki fragments. We now know that a diverse range of bacterial species possess up to six of these accessory bacterial ATP-dependent DNA ligases (b-ADLs), which vary in size and enzymatic domain associations. Here we compare the domain structure of different types of b-ADLs and investigate their distribution among the bacterial domain to describe possible evolutionary trajectories that gave rise to the sequence and structural diversity of these enzymes. Previous biochemical and genetic analyses have delineated three main classes of these enzymes: Lig B, Lig C and Lig D, which appear to have descended from a common ancestor within the bacterial domain. In the present study, we delineate a fourth group of b-ADLs, Lig E, which possesses a number of unique features at the primary and tertiary structural levels. The biochemical characteristics, domain structure and inferred extracellular location sets this group apart from the other b-ADLs. The results presented here indicate that the Lig E type ligases were horizontally transferred into bacteria in a separate event from other b-ADLs possibly from a bacteriophage.