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New NodW- or NifA-Regulated Bradyrhizobium japonicum Genes

April 2003 , Volume 16 , Number  4
Pages  342 - 351

Isabelle Caldelari Baumberger , 1 Nicole Fraefel , 1 Michael Göttfert , 2 and Hauke Hennecke 1

1Institut für Mikrobiologie, Eidgenössische Technische Hochschule, CH-8092 Zürich, Switzerland; 2Institut für Genetik, Technische Universität Dresden, D-01062 Dresden, Germany

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Accepted 18 December 2002.

A cluster of genes coding for putative plant cell-wall degrading enzymes (i.e., genes for two endoglucanases [gunA and gunA2], one pectinmethylesterase [pme], and one polygalacturonase [pgl]) was identified by sequence similarities in the symbiotic region of the Bradyrhizobium japonicum chromosome. In addition, a systematic screen of the region revealed several genes potentially transcribed by the σ54-RNA polymerase and activated by the transcriptional regulator NifA (i.e., genes for proteins with similarity to outer membrane proteins [id117 and id525] and a citrate carrier [id331 or citA] and one open reading frame without similarity to known proteins [id747]). Expression studies using transcriptional lacZ fusions showed that gunA2 and pgl were strongly induced by the isoflavone genistein in a NodW-dependent manner, suggesting a role of the gene products in early events of the nodulation process; by contrast, gunA and pme expression was very weak in the conditions tested. The gunA2 gene product was purified and was shown to have cellulase activity. β-Galactosidase activity expressed from transcriptional lacZ fusions to id117, id525, and id747 in the wild type and in nifA and rpoN mutant backgrounds confirmed that their transcription was dependent on NifA and σ54. Despite the presence of a -24/-12-type promoter and a NifA binding site upstream of citA, no regulation could be demonstrated in this case. Null mutations introduced in gunA, gunA2, pgl, pme, citA, id117, id525, and id747 did not impair the symbiosis with the host plants.

Additional keywords: nitrogen fixation, nod genes.

© 2003 The American Phytopathological Society