Characterization of the mechanisms behind the establishment of curved cell shape in Rhodospirillum rubrum

Bacteria come in many different shapes, which help them to move efficiently in the ecological niches they inhabit and thus ensure optimal fitness and survival. The mechanisms underlying morphogenesis in curved bacteria is a particularly interesting field of research, because in order to achieve a curved shape, the symmetry of the cells needs to be broken. Previous studies of curved model organisms, such as Vibrio cholerae, Helicobacter pylori and Caulobacter crescentus, have revealed that bacteria have evolved a variety of fundamentally different morphogenesis mechanisms. This variety in mechanisms behind the establishment of curved cell shape urged us to characterize the morphogenesis system of a so far poorly investigated group of curved bacteria. In our analysis of the morphogenetic pathway of the alpha-proteobacterium Rhodospirillum rubrum, we have identified and characterized multiple proteins involved in the establishment and maintenance of its curved cell shape. Some of them, including spatially organized porins in the outer membrane that position the conserved peptidoglycan-associated lipoprotein PapS, are crucial for the establishment of cell curvature. Together they form a dense and static structure that mediates the asymmetric localization of the cell wall-forming elongasome enzyme complex, which appears to be the driving enzymatic force behind curved cell shape establishment in R. rubrum. We also report additional PapS-dependent and -independent morphogenetic mechanisms that are important for curvature fine-tuning. The interplay of these systems ensures an optimal but adjustable degree of curvature in R. rubrum, which we hypothesize could be beneficial under specific environmental conditions in which an altered curvature increases the fitness of the cells. Together, these factors form a novel and, in large parts, conserved morphogenetic protein network that extends from the cytoplasm to the outer membrane and likely mediates the curved cell morphology in many spiral-shaped members of the family Rhodospirillaceae. These results give further insight into basic principles of cellular organization in curved bacteria and therefore could provide the foundation for the development of new antibiotics.