Symbiosis between rhizobia and legumes is initiated by an exchange of signal molecules. Communication starts with flavonoids that are secreted by the host plant via the root system. These flavonoids lead to the activation of the transcriptional regulator NodD, a member of the LysR family. NodD binds to the nod box, a conserved sequence within the promoter of nodulation genes, and initiates their transcription (Schlaman et al. 1998). In Bradyrhizobium japonicum 110, a second regulatory system consisting of the genes nodV and nodW is involved in nod gene regulation (Göttfert et al. 1990; Loh et al. 1997). These genes are indispensable for the infection of several host plants. The Nod proteins catalyze the synthesis of Nod factors, modified lipochito-oligosaccharides, that are secreted by the bacterial cell. These signal molecules induce the first steps of nodule development (Hadri, Bisseling 1998).
Little is known about alternative signaling pathways that may eventually also influence later stages in nodule development. For Sinorhizobium fredii, it was shown that flavonoids stimulate the secretion of proteins (Krishnan, Pueppke 1993). Sequence analysis of the closely related Rhizobium sp. strain NGR234 (Freiberg et al. 1997) and of Mesorhizobium loti (Kaneko et al. 2000) indicated the presence of a type III secretion system. Many bacterial pathogens of plants and animals use such systems for the export of proteins (Hueck 1998). In several cases, it was demonstrated that these proteins enter the host cell. In NGR234, two secreted proteins have been identified (Viprey et al. 1998). Mutations in the type III secretion system affect symbiosis depending on the host (Meinhardt et al. 1993; Viprey et al. 1998).
Recently, sequence analysis of a 410- kb chromosomal region of B. japonicum revealed the presence of a type III secretion system (Göttfert et al. 2001). In this work, the gene cluster is referred to as tts (type three secretion). Here, we present the results of mutational, transcriptional and further nucleotide sequence analyses of this region.
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