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Investigation of the role of the BAM complex and SurA chaperone in outer membrane protein biogenesis and T3SS expression in Salmonella

INRA

ABSTRACT

In Escherichia coli, the assembly of outer membrane proteins (OMP) requires the BAM complex and periplasmic chaperones such as SurA or DegP. Previous works have suggested a potential link between OMP assembly and Type-III secretion system (T3SS) expression. In order to test this hypothesis, we studied the role of the different lipoproteins of the BAM complex (i.e. BamB, BamC, BamD and BamE) and the periplasmic chaperones SurA and DegP in these two phenotypes in Salmonella. Analysis of the corresponding deletion mutants showed that as previously described with the bamB mutant, BamD, SurA and to a lesser extent BamE play a role in OM biogenesis in S. Enteritidis while the membrane was not notably disturbed in bamC and degP mutants. Interestingly, we found that BamD is not essential in Salmonella, unlike its homolog in E. coli and in Neisseria gonorrhoeae. By contrast, BamD was the only protein required for full expression of T3SS-1 and flagella as demonstrated by transcriptional analysis of the genes involved in the biosynthesis of these T3SS. In this line, bamD mutants showed a reduced secretion of effector proteins by these T3SS and a reduced invasion rate in HT-29 cells. As surA and bamE mutants had lower levels of OMP in their OM but were not altered in T3SS-1 and flagella expression, these results demonstrate the absence of a link between an OMP assembly defect and the down-regulation of T3SS in Salmonella, which seems therefore to be related to a more specific mechanism involving at least BamB and BamD.

¹ E-mail: ipayant{at}tours.inra.fr

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