L egionella pneumophila utilizes a single‐player disulfide‐bond oxidoreductase system to manage disulfide bond formation and isomerization

L egionella pneumophila uses a single homodimeric disulfide bond ( DSB ) oxidoreductase DsbA 2 to catalyze extracytoplasmic protein folding and to correct DSB errors through protein‐disulfide isomerase ( PDI ) activity. I n E scherichia coli , these functions are separated to avoid futile cycling. In L . pneumophila , DsbA 2 is maintained as a mixture of disulfides ( S ‐ S ) and free thiols ( SH ), but when expressed in E . coli , only the SH form is observed. We provide evidence to suggest that structural differences in DsbB oxidases ( LpDsbB 1 and LpDsbB 2) and DsbD reductases ( LpDsbD 1 and LpDsbD 2) (compared with E . coli ) permit bifunctional activities without creating a futile cycle. L p dsbB1 and L p dsbB2 partially complemented an Ec dsbB mutant while neither L p dsbD1 nor L p dsbD2 complemented an Ec dsbD mutant unless DsbA 2 was also expressed. When the dsb genes of E . coli were replaced with those of L . pneumophila , motility was restored and DsbA 2 was present as a mixture of redox forms. A dominant‐negative approach to interfere with DsbA 2 function in L . pneumophila determined that DSB oxidase activity was necessary for intracellular multiplication and assembly/function of the D ot/ I cm T ype IVb secretion system. Our studies show that a single‐player system may escape the futile cycle trap by limiting transfer of reducing equivalents from LpDsbDs to DsbA 2.