HrpB2 and HrpF from Xanthomonas are type III-secreted proteins and essential for pathogenicity and recognition by the host plant

The interaction between the plant pathogen Xanthomonas campestris pv, vesicatoria and its host plants is controlled by hrp genes (hypersensitive reaction and pathogenicity), which encode a type ill protein secretion system, Among type ill-secreted proteins are avirulence proteins, effecters involved in the induction of plant defence reactions, Using nonpolar mutants, we investigated the role of 12 hrp genes in the secretion of the avirulence protein AvrBs3 from X. c, pv, vesicatoria and a heterologous protein, YopE, from Yersinia pseudotuberculosis. Genes conserved among type III secretion systems (hrcQ, hrcR, hrcS and hrcT) as well as non-conserved genes (hrpB1, hrpB2, hrpB4, hrpB5, hrpD5 and hrpD6) were shown to be required for secretion. Protein localization studies using specific antibodies showed that HrpB1 and HrpB4, as well as the putative ATPase HrcN, were mainly found in the soluble fraction of the bacterial cell, in contrast, HrpB2 and HrpF, which is related to NoIX of Rhizobium fredii, are secreted into the culture medium in an hrp-dependent manner, As HrpB2, but not HrpF, is essential for type ttl protein secretion, there might be a hierarchy in the secretion process. We propose that HrpF, which is dispensable for protein secretion but required for AvrBs3 recognition in planta, functions as a translocator of effector proteins into the host cell.