ABSTRACT
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
(σ22), 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.
The algT Gene of Pseudomonas syringae pv. glycinea andNew Insights into the Transcriptional Organization of thealgT-muc Gene Cluster