Sigma factors allow bacteria to maintain basal gene expression while at the same time differentially regulating the expression of specific groups of genes in response to altered environmental or developmental signals. A subfamily of extracytoplasmic function (ECF) sigma factors are found in a great diversity of bacteria, many of which activate the transcription of their regulons in response to change in environmental conditions. For example, ECF sigma factors regulate iron uptake, alginate biosynthesis and can activate regulatory cascades involved in the control expression of hrp genes (“hypersensitive response and pathogenicity” ) from the type III secretion system (TTSS). In Ralstonia solancearum this activation requires physical interaction between the bacteria and plant cell, which is sensed by the outer membrane receptor PrhA. This interaction triggers a complex regulatory signal transduction cascade integrated by the PrhR (anti-σ), PrhI (s^ECF), PrhJ, HrpG and HrpB regulators. In the genome of Xanthomonas axonopodis pv citri (Xac) genes homologs to the above mentioned Ralstonia solancearum regulators have been identified, but their functions are unknown. We have selected to study the genes Xac 2191 and Xac 4129, that encode PrhI homologs, and the genes Xac 2192 and Xac 4130 that encode PrhR homologs. We have analysed protein-protein interactions involving two sigma factors (Xac 2191 and Xac 4129) by yeast two-hybrid assays employing a Xac genomic DNA prey library. We have cloned, expressed and purified the above four proteins in E.coli. We are evaluating their interaction “in vitro” by overlay and pulldown experiments. These studies will allow us to elucidate the genetic and signal transduction pathways in which they are involved, and also to understand their role, if any with the activation of Xanthomonas axonopodis pv citri hrp gene expression.