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Journal of Bacteriology, August 1999, p. 4790-4797, Vol. 181, No. 16
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

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

Karin van Dijk,1 Derrick E. Fouts,2 Amos H. Rehm,2 Angela R. Hill,1 Alan Collmer,2 and James R. Alfano1,*

Department of Biological Sciences, University of Nevada, Las Vegas, Nevada 89154-4004,1 and Department of Plant Pathology, Cornell University, Ithaca, New York 14583-42032

Received 12 April 1999/Accepted 4 June 1999

We present here data showing that the Avr proteins HrmA and AvrPto are secreted in culture via the native Hrp pathways from Pseudomonas syringae pathovars that produce these proteins. Moreover, their secretion is strongly affected 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°C and when the culture medium was pH 6.0. In contrast, temperature 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 suggests that there are at least two classes of proteins that travel the P. syringae 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 detected in lower amounts in culture supernatants under the appropriate conditions. Because HrmA was shown to be a Hrp-secreted protein, we have changed the name of hrmA to hopPsyA to reflect that it encodes a Hrp outer protein from P. syringae pv. syringae. The functional P. syringae Hrp cluster encoded by cosmid pHIR11 conferred upon P. fluorescens but not Escherichia coli the ability to secrete HopPsyA in culture. The use of these optimized conditions should facilitate the identification of additional proteins traveling the Hrp pathway and the signals that regulate this protein traffic.

* Corresponding author. Mailing address: Department of Biological Sciences, University of Nevada, Las Vegas, 4505 Maryland Parkway, Box 454004, Las Vegas, NV 89154-4004. Phone: (702) 895-4420. Fax: (702) 895-3956. E-mail: alfanoj{at}nevada.edu.

Journal of Bacteriology, August 1999, p. 4790-4797, Vol. 181, No. 16
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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