The Escherichia coli trmE (mnmE) gene, involved in tRNA modification, codes for an evolutionarily conserved GTPase with unusual biochemical properties

The evolutionarily conserved 50K protein of Escherichia coli, encoded by o4 54, contains a consensus GTP-binding motif, Here we show that 50K is a GTPa se that differs extensively from regulatory GTPases such as p21, Thus, 50K exhibits a very high intrinsic GTPase hydrolysis rate, rather low affinity for GTP, and extremely low affinity for GDP, Moreover, it can form self-ass emblies. Strikingly, the 17 kDa GTPase domain of 50K conserves the guanine nucleotide-binding and GTPase activities of the intact 50K molecule. Theref ore, the structural requirements for GTP binding and GTP hydrolysis by 50K are without precedent and justify a separate classification in the GTPase s uperfamily, Immunoelectron microscopy reveals that 50K is a cytoplasmic pro tein partially associated with the inner membrane. We prove that o454 is al lelic with trmE, a gene involved in the biosynthesis of the hypermodified n ucleoside 5-methylaminomethyl-2-thiouridine, which is found in the wobble p osition of some tRNAs, Our results demonstrate that 50K is essential for vi ability depending on the genetic background. We propose that combination of mutations affecting the decoding process, which separately do not reveal a n obvious defect in growth, can give rise to lethal phenotypes, most likely due to synergism.