RESUMEN
We have sequenced two genes dctB and dctD required for the activation of the C4-dicarboxylate transport structural gene dctA in free-living Rhizobium leguminosarum. The hydropathic profile of the dctB gene product (DctB) suggested that its N-terminal region may be located in the periplasm and its C-terminal region in the cytoplasm. The C-terminal region of DctB was strongly conserved with similar regions of the products of several regulatory genes that may act as environmental sensors, including ntrB, envZ, virA, phoR, cpxA, and phoM. The N-terminal domains of the products of several regulatory genes thought to be transcriptional activators, including ntrC, ompR, virG, phoB and sfrA. In addition, the central and C-terminal regions of DctD were strongly conserved with the products of ntrC and nifA, transcriptional activators that require the alternate sigma factor rpoN (ntrA) as co-activator. The central region of DctD also contained a potential ATP-binding domain. These results are consistent with recent results that show that rpoN product is required for dctA activation, and suggest that DctB plus DctD-mediated transcriptional activation of dctA may be mechanistically similar to NtrB plus NtrC-mediated activation of glnA in E. coli.
Asunto(s)
Ácidos Dicarboxílicos/metabolismo , Genes Bacterianos , Genes Reguladores , Fijación del Nitrógeno/genética , Rhizobium/genética , Secuencia de Aminoácidos , Secuencia de Bases , Transporte Biológico , Modelos Biológicos , Datos de Secuencia Molecular , Rhizobium/metabolismo , Homología de Secuencia de Ácido NucleicoRESUMEN
Cosmids containing C4-dicarboxylate transport (dct) genes were identified from a gene bank of Rhizobium leguminosarum DNA made in the broad-host-range vector pLAFR1 by their ability to complement R. trifolii dct mutants. The dct genes were further characterized by subcloning, restriction site mapping, and transposon Tn5 and Tn7 mutageneses. Three dct loci were identified within a 5.5-kilobase region of DNA, in the order dctA-dctB-dctC. The results suggested that dctA encoded a structural component necessary for C4-dicarboxylate transport, whereas dctB and dctC encoded positive regulatory elements, and that dctA was transcribed divergently from dctB and dctC. Expression of dctA and dctC was obtained from vector promoters in some pLAFR1- and pSUP106-based plasmids.