Identification of Genes in the σ
22
Regulon of Pseudomonas aeruginosa Required for Cell Envelope Homeostasis in Either the Planktonic or the Sessile Mode of Growth
Authors: Wood Lynn F. , Ohman Dennis E. 
Published: May 2012

The
Pseudomonas aeruginosa
extracytoplasmic functioning (ECF) sigma factor σ
22
is encoded by
algT/algU
and is inhibited by anti-sigma factor MucA. σ
22
was originally discovered for its essential role in the expression of the exopolysaccharide alginate by mucoid strains associated with chronic pulmonary infection. However, σ
22
is now known to also have a large regulon associated with the response to cell wall stress. Our recent transcriptome analysis identified 293 open reading frames (ORFs) in the σ
22
stress stimulon that include genes for outer envelope biogenesis and remodeling, although most of the genes have undefined functions. To better understand the σ
22
-dependent stress response, mutants affected in 27 genes of the σ
22
stimulon were examined and expression was studied with
lacZ
fusions. Mutants constructed in the 27 genes showed no major change in response to cell wall-acting antibiotics or growth at elevated temperatures nor in alginate production. The mutants were examined for their effects on the expression of the σ
22
-dependent promoter of the alginate biosynthetic operon (P
algD
) as a measure of σ
22
derepression from MucA. By testing P
algD
expression under both planktonic and sessile growth conditions, 11 genes were found to play a role in the stress response that activates σ
22
. Some mutations caused an increase or a decrease in the response to cell wall stress. Interestingly, mutations in 7 of the 11 genes caused constitutive P
algD
expression under nonstressed conditions and thus showed that these genes are involved in maintaining envelope homeostasis. Mutations in PA0062 and PA1324 showed constitutive P
algD
expression during both the planktonic and the sessile modes of growth. However, the PA5178 mutation caused constitutive P
algD
expression only during planktonic growth. In contrast, mutations in PA2717, PA0567, PA3040, and PA0920 caused constitutive P
algD
expression only in the sessile/biofilm mode of growth. This provides evidence that the σ
22
stimulon for cell envelope homeostasis overlaps with biofilm control mechanisms.

IMPORTANCE
During chronic lung infections, such as in cystic fibrosis patients,
Pseudomonas aeruginosa
produces the exopolysaccharide alginate and forms biofilms that shield the organisms from the immune response and increase resistance to antibiotics. Activation of alginate genes is under the control of an extracytoplasmic stress response system that releases an alternative sigma factor (σ
22
) in response to cell wall stress and then activates expression of a large regulon. In this study, a mutant analysis of 27 members of the regulon showed that 11 play a role in envelope homeostasis and affect the stress response system itself. Interestingly, some genes demonstrate effects only in either the planktonic (free-swimming) or the sessile (biofilm) mode of growth, which leads to persistence and antibiotic tolerance. The studies presented here provide an important initial step in dissecting the mechanisms that regulate a critical signal transduction pathway that impacts
P. aeruginosa
pathogenesis.