The Avr (effector) proteins HrmA (HopPsyA) and AvrPto are secreted in culture from Pseudomonas syringae pathovars via the Hrp (type III) protein secretion system in a temperature- and pH-sensitive manner

We present here data showing that the Avr proteins HrmA and AvrPto are secr eted in culture via the native Hrp pathways from Pseudomonas syringae patho vars that produce these proteins, Moreover, their secretion is strongly aff ected by the temperature and pH of the culture medium. Both HrmA and AvrPto were secreted at their highest amounts when the temperature was between 18 and 22 degrees C and when the culture medium was pH 6.0. In contrast, temp erature did not affect the secretion of HrpZ. pH did affect HrpZ secretion, but not as strongly as it affected the secretion of HrmA. This finding sug gests that there are at least two classes of proteins that travel the P. sy ringae pathway: putative secretion system accessory proteins, such as HrpZ, which are readily secreted in culture; and effector proteins, such as HrmA and AvrPto, which apparently are delivered inside plant cells and are dete cted in lower amounts in culture supernatants under the appropriate conditi ons. Because HrmA was shown to be a Hrp-secreted protein, we hare changed t he name of hrmA to hopPsyA to reflect that it encodes a Hrp outer protein f rom P. syringae pv. syringae. The functional P. syringae Hrp cluster encode d by cosmid pHIR11 conferred upon P. fluorescens but not Escherichia coli t he ability to secrete HopPsyA in culture. The use of these optimized condit ions should facilitate the identification of additional proteins traveling the Hrp pathway and the signals that regulate this protein traffic.