The Caulobacter crescentus outer membrane protein Omp58 (RsaF) is not required for paracrystalline S-layer secretion

Caulobacter crescentus intrinsic dimorphism provides a prompt bimodal response to copper stress

Stress response to fluctuating environments often implies a time-consuming reprogramming of gene expression. In bacteria, the so-called bet hedging strategy, which promotes phenotypic stochasticity within a cell population, is the only fast stress response described so far(1). Here, we show that Caulobacter crescentus asymmetrical cell division allows an immediate bimodal response to a toxic metals-rich environment by allocating specific defence strategies to morphologically and functionally distinct siblings. In this context, a motile swarmer cell favours negative chemotaxis to flee from a copper source, whereas a sessile stalked sibling engages a ready-to-use PcoAB copper homeostasis system, providing evidence of a prompt stress response through intrinsic bacterial dimorphism.

Two outer membrane proteins are required for maximal type I secretion of the Caulobacter crescentus S-layer protein

Transport of RsaA, the crystalline S-layer subunit protein of Caulobacter crescentus, is mediated by a type I secretion mechanism. Two proteins have been identified that play the role of the outer membrane protein (OMP) component in the RsaA secretion machinery. The genes rsaF(a) and rsaF(b) were identified by similarity to the Escherichia coli hemolysin secretion OMP TolC by using the C. crescentus genome sequence. The rsaF(a) gene is located several kilobases downstream of the other transporter genes, while rsaF(b) is completely unlinked. An rsaF(a) knockout had approximately 56% secretion compared to wild-type levels, while the rsaF(b) knockout reduced secretion levels to approximately 79%. When expression of both proteins was eliminated, there was no RsaA secretion, but a residual level of approximately 9% remained inside the cell, suggesting posttranslational autoregulation. Complementation with either of the individual rsaF genes by use of a multicopy vector, which resulted in 8- to 10-fold overexpression of the proteins, did not restore RsaA secretion to wild-type levels, indicating that both rsaF genes were required for full-level secretion. However, overexpression of rsaF(a) (with normal rsaF(b) levels) in concert with overexpression of rsaA resulted in a 28% increase in RsaA secretion, indicating a potential for significantly increasing expression levels of an already highly expressing type I secretion system. This is the only known example of type I secretion requiring two OMPs to assemble a fully functional system.