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Mutational and Transcriptional Analysis of the Type III Secretion System of Bradyrhizobium japonicum

December 2002 , Volume 15 , Number  12
Pages  1,228 - 1,235

Andrea Krause , Anke Doerfel , and Michael Göttfert

Institut für Genetik, Technische Universität Dresden, Mommsenstraße 13, 01062 Dresden, Germany

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Accepted 9 August 2002.

Sequencing the symbiotic region of Bradyrhizobium japonicum revealed a gene cluster (tts) encoding a type III secretion system (TTSS) that is similar to those found in Mesorhizobium loti MAFF303099 and Rhizobium strain NGR234. In addition to genes that are likely to encode structural core components of the TTSS, the cluster contains several open reading frames that are found exclusively in rhizobia or that are specific to B. japonicum. Depending on the host, mutations within this cluster affected nodulation capacity to different extents. One of the genes likely encodes a transcriptional activator (TtsI) of the two-component regulatory family. Upstream of ttsI, a nod box promoter was identified. Expression of ttsI could be induced by genistein. This induction depended on the transcriptional activator protein NodW as well as the nodD1nodD2nolA gene region. TtsI was found to be involved in transcriptional regulation of the tts gene cluster. Sequence comparison revealed a conserved tts box element within putative promoter regions of several genes. Here, we propose a model of the regulatory cascade leading to the induction of the tts gene cluster.

Additional keywords: nitrogenase activity , symbiosis .

© 2002 The American Phytopathological Society