Two novel proteins, PopB, which has functional nuclear localization signals, and PopC, which has a large leucine-rich repeat domain, are secreted through the hrp-secretion apparatus of Ralstonia solanacearum.

The Ralstonia solanacearum hrp gene cluster codes for components of a type III secretion pathway necessary for the secretion of PopA1, a hypersensitive response-like elicitor protein. In the present study, we show that several other Hrp-secreted proteins can be detected by growing wild-type bacteria in minimal medium in the presence of Congo red. Two of these proteins, PopB and PopC, are encoded by genes located downstream of popA and constitute an operon with popA. popABC mutants retain the wild-type ability to cause disease in hosts and to elicit the hypersensitive response on non-hosts. Expression of the popABC operon is controlled by the hrpB regulatory gene and is induced upon co-culture with Arabidopsis cell suspensions. This plant cell-specific induction depends on PrhA, a putative receptor for plant specific signal(s). The transcription of the popABC operon is not modified by the addition of Congo red to the growth medium and the intracellular pools of PopB and PopC are very similar in the absence or presence of Congo red. Preliminary data suggest that Congo red stabilizes secreted proteins in the extracellular medium. PopB is a 173-amino-acid-basic protein that contains a functional bipartite nuclear localization signal. PopC is a 1024-amino-acid protein that carries 22 tandem leucine-rich repeats (LRR). The LRR domain of this protein forms a consensus that perfectly matches the predicted eukaryotic cytoplasmic LRR consensus. We propose that PopB and PopC may be translocated into plant cells via the Hrp pathway.