Temperature-dependent expression of type III secretion system genes and its regulation in bradyrhizobium japonicum

The genome-wide expression profiles of Bradyrhizobium japonicum in response to soybean (Glycine max (L.) Merr.) seed extract (SSE) and genistein were monitored with time at a low temperature (15°C). A comparison with the expression profiles of the B. japonicum genome previously captured at the common growth temperature (30°C) revealed that the expression of SSE preferentially induced genomic loci, including a large gene cluster encoding the type III secretion system (T3SS), were considerably delayed at 15°C, whereas most nodulation (nod) gene loci, including nodD1 and nodW, were rapidly and strongly induced by both SSE and genistein. Induction of the T3SS genes was progressively activated upon the elevation of temperature to 30°C and positively responded to culture population density. In addition, genes nolA and nodD2 were dramatically induced by SSE, concomitantly with the expression of T3SS genes. However, the deletion mutation of nodD2 but not nolA led to elimination of the T3SS genes expression. These results indicate that the expression of the T3SS gene cluster is tightly regulated with integration of environmental cues such as temperature and that NodD2 may be involved in its efficient induction in B. japonicum.