1.6.15
A COMPLEX REGULATORY CASCADE CONTROLS RALSTONIA SOLANACEARUM HRP GENE EXPRESSION IN RESPONSE TO PLANT-SPECIFIC SIGNALS

B BRITO, M MARENDA, M ARLAT, S GENIN and C BOUCHER

Laboratoire de Biologie Moleculaire des Relations Plantes-Microorganismes, INRA-CNRS, 31326 Castanet-Tolosan cedex, France

Background and objectives
Ralstonia solanacearum is the causal agent of the bacterial wilt disease that affects many agronomically important crops in tropical and subtropical areas, and that has been recently detected in several European countries. As in many other plant pathogenic bacteria, hrp genes in R. solanacearum control the ability both to elicit disease on compatible hosts and to trigger the hypersensitive response (HR) on non-host plants. Homologues of some hrp genes are also found in diverse animal pathogenic bacteria. These common genes are involved in the assembly of a secretion machinery of proteins called the type III secretion system [1].

The hrp gene cluster of strain GM1000 is organized in at least five transcriptional units, whose expression is induced in minimal medium and in planta by the hrpB regulatory gene. Recently, we have shown that upon co-culture with plant cell suspensions, expression of the hrp transcriptional units is strongly induced. This activation is dependent on the product of the prhA gene which is located in the left-hand region of the hrp gene cluster. PrhA displays significant similarities with members of the family of TonB-dependent outer membrane siderophore receptors. Despite the similarities observed with this class of receptors, neither prhA nor hrp genes are regulated by iron status, so we have postulated that PrhA could act as a receptor for an unidentified plant signal(s), which would be transduced to the hrpB gene by a novel regulatory pathway [2].

Results and conclusions
The genetic characterization of the right-flanking region of the hrp gene cluster has led to the identification of four new regulatory genes, named hrpG, prhJ, prhI and prhR, which are involved in this regulatory cascade. HrpG and PrhJ showed similarities to response-regulator proteins belonging to the two-component signal-transduction systems. prhI and prhR encode, respectively, an extracytoplasmic sigma factor and a transmembrane protein. With the exception of hrpG, which is homologous to the hrpG gene of Xanthomonas campestris, none of the three other genes was found to be homologous to other hrp regulatory genes described to date. Our results demonstrate that these four genes control both hrpB gene expression upon co-culture with plant-cell suspensions, and the transduction of the plant-specific signals sensed by PrhA. A model dissecting this novel regulatory cascade that controls hrp gene expression in R. solanacearum is presented.

References
1. Van Gijsegem F, Gough C, Zischek C et al., 1995. Molecular Microbiology 15, 1095-1114.
2. Marenda M, Brito B, Callard D et al., 1998. Molecular Microbiology 27, 437-454.