The wobble nucleoside 5-methylaminomethyl-2-thio-uridine (mnm(5)s(2)U) is present in bacterial tRNAs specific for Lys and Glu and 5-carboxymethylaminomethyl-2-thio-uridine (cmnm(5)s(2)U) in tRNA specific for Gln. The sulfur of (c) mnm(5)s(2)U may be exchanged by selenium (Se)-a reaction catalyzed by the selenophosphate-dependent tRNA 2-selenouridine synthase encoded by the mnmH (ybbB, selU, sufY) gene. The MnmH protein has a rhodanese domain containing one catalytic Cys (C97) and a P-loop domain containing a Walker A motif, which is a potential nucleotide binding site. We have earlier isolated a mutant of Salmonella enterica, serovar Typhimurium with an alteration in the rhodanese domain of the MnmH protein (G67E) mediating the formation of modified nucleosides having a geranyl (ge)-group (C10H17-fragment) attached to the s(2) group of mnm(5)s(2)U and of cmnm(5)s(2)U in tRNA. To further characterize the structural requirements to increase the geranylation activity, we here report the analysis of 39 independently isolated mutants catalyzing the formation of mnm(5)ges(2)U. All these mutants have amino acid substitutions in the rhodanese domain demonstrating that this domain is pivotal to increase the geranylation activity. The wild type form of MnmH(+) also possesses geranyltransferase activity in vitro although only a small amount of the geranyl derivatives of (c) mnm(5)s(2)U is detected in vivo. The selenation activity in vivo has an absolute requirement for the catalytic Cys97 in the rhodanese domain whereas the geranylation activity does not. Clearly, MnmH has two distinct enzymatic activities for which the rhodanese domain is pivotal. An intact Walker motif in the P-loop domain is required for the geranylation activity implying that it is the binding site for geranylpyrophosphate (GePP), which is the donor molecule in vitro in the geranyltransfer reaction. Purified MnmH from wild type and from the MnmH(G67E) mutant have bound tRNA, which is enriched with geranylated tRNA. This in conjunction with earlier published data, suggests that this bound geranylated tRNA may be an intermediate in the selenation of the tRNA.