Expression of the Bradyrhizobium japonicum type III secretion system in legume nodules and analysis of the associated tts box promoter

In Bradyrhizobium japonicum, as in some other rhizobia, symbiotic efficiency is influenced by a type III secretion system (T3SS). Most genes encoding the transport machinery and secreted proteins are preceded by a conserved 30-bp motif, the type-three secretion (tts) box. In this study, we found that regions downstream of 34 tts boxes are transcribed. For nopB, nopL, and gunA2, the transcriptional start sites were found to be 12, 11, and 10 bp downstream of their tts boxes, respectively. The deletion of this motif or modification of two or more conserved residues strongly reduced expression of nopB. This indicates that the Us box is an essential promoter element. Data obtained with lacZ reporter gene fusions of five genes preceded by a tts box (gunA2, nopB, rhcV, nopL, and blr1806) revealed that they are expressed in 4-week-old nodules of Macroptilium atropurpureum. These data suggest that the T3SS is active in mature nitrogen-fixing nodules. The two-component response regulator TtsI is required for the expression of rhcV, nopL, and blr1806 in bacteroids. Staining of inoculated roots showed that nopB is also expressed in early infection stages.