Bacteria encode a number of relatively poorly characterized GTPases, including the essential, ribosome-associated Obg/CgtA proteins. In contrast to Ras-like proteins, it appears that the Obg/CgtA proteins bind guanine nucleotides with modest affinity and hydrolyze GTP relatively slowly. We show here that the Vibrio harveyi CgtA(V) exchanges guanine nucleotides rapidly and has a modest affinity for nucleotides, suggesting that these features are a universal property of the Obg/CgtA family. Interestingly, CgtA(V) possesses a significantly more rapid GTP hydrolysis rate than is typical of other family members, perhaps reflecting the diversity and specificity of bacterial ecological niches. (c) 2005 Elsevier Inc. All rights reserved.