Conserved P-loop GTPases of unknown function in bacteria: an emerging and vital ensemble in bacterial physiology

Establishing the roles of conserved gene products in bacteria is of fundamental importance to our understanding of the core protein complement necessary to sustain cellular life. P-loop GTPases and related ATPases represent an abundant and remarkable group of proteins in bacteria that, in many cases, have evaded characterization. Here, efforts aimed at understanding the cellular function of a group of 8 conserved, poorly characterized genes encoding P-loop GTPases, era, obg, trmE, yjeQ, engA, yihA, hflX, ychF, and a related ATPase, yjeE, are reviewed in considerable detail. While concrete cellular roles remain elusive for all of these genes and considerable pleiotropy has plagued their study, experiments to date have frequently implicated the ribosome. In the case of era, obg, yjeQ, and engA, the evidence is most consistent with roles in ribosome biogenesis, though the prediction is necessarily putative. While the protein encoded in trmE clearly has a catalytic function in tRNA modification, the participation of its GTPase domain remains obscure, as do the functions of the remaining proteins. A full understanding of the cellular functions of all of these important proteins remains the goal of ongoing studies of cellular phenotype and protein biochemistry.