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The algT Gene of Pseudomonas syringae pv. glycinea and New Insights into the Transcriptional Organization of the algT-muc Gene Cluster

Alexander Schenk,¹ Michael Berger,¹ Lisa M. Keith,^2, Carol L. Bender,² Georgi Muskhelishvili,¹ and Matthias S. Ullrich^1*

School of Engineering and Sciences, Campus Ring 1, International University Bremen, D-28759 Bremen, Germany,¹ Department of Entomology and Plant Pathology, Oklahoma State University, 127 Noble Research Center, Stillwater, Oklahoma 74078²

Received 29 July 2006/ Accepted 31 August 2006

The phytopathogenic bacterium Pseudomonas syringae pv. glycinea infects soybean plants and causes bacterial blight. In addition to P. syringae, the human pathogen Pseudomonas aeruginosa and the soil bacterium Azotobacter vinelandii produce the exopolysaccharide alginate, a copolymer of D-mannuronic and L-guluronic acids. Alginate production in P. syringae has been associated with increased fitness and virulence in planta. Alginate biosynthesis is tightly controlled by proteins encoded by the algT-muc regulatory gene cluster in P. aeruginosa and A. vinelandii. These genes encode the alternative sigma factor AlgT (²²), its anti-sigma factors MucA and MucB, MucC, a protein with a controversial function that is absent in P. syringae, and MucD, a periplasmic serine protease and homolog of HtrA in Escherichia coli. We compared an alginate-deficient algT mutant of P. syringae pv. glycinea with an alginate-producing derivative in which algT is intact. The alginate-producing derivative grew significantly slower in vitro growth but showed increased epiphytic fitness and better symptom development in planta. Evaluation of expression levels for algT, mucA, mucB, mucD, and algD, which encodes an alginate biosynthesis gene, showed that mucD transcription is not dependent on AlgT in P. syringae in vitro. Promoter mapping using primer extension experiments confirmed this finding. Results of reverse transcription-PCR demonstrated that algT, mucA, and mucB are cotranscribed as an operon in P. syringae. Northern blot analysis revealed that mucD was expressed as a 1.75-kb monocistronic mRNA in P. syringae.

Published ahead of print on 29 September 2006.

Present address: USDA-ARS, Pacific Basin Agricultural Research Center, Tropical Plant Genetic Resource Management Unit, P.O. Box 4487, Hilo, HI 96720.

This article has been cited by other articles:

• Schenk, A., Weingart, H., Ullrich, M. S. (2008). The alternative sigma factor AlgT, but not alginate synthesis, promotes in planta multiplication of Pseudomonas syringae pv. glycinea. Microbiology 154: 413-421 [Abstract] [Full Text]