The structure of a Bacillus subtilis YphC/GDP complex shows that it contains two GTPase domains that pack against a central domain whose fold resembles that of an RNA binding KH-domain. Comparisons of this structure to that of a homologue in Thermotoga maritima reveals a dramatic rearrangement in the position of the N-terminal GTPase domain with a shift of up to 60 A and the formation of a totally different interface to the central domain. This rearrangement appears to be triggered by conformational changes of the switch II region in this domain in response to nucleotide binding. Modeling studies suggest that this motion represents transitions between the "on" and "off" states of the GTPase, the effect of which is to alternately expose and bury a positively charged face of the central domain that we suggest is involved in RNA recognition as part of the possible role of this enzyme in ribosome binding.