Cellular organization, division, and differentiation in an ancient, genetically reduced bacterium

PROJECT SUMMARY / ABSTRACT In this proposal we aim to further define and characterize the molecular mechanisms that enable division, cell- size regulation, and differentiation in members of the Chlamydiales: a bacterial order that consists entirely of species that live exclusively in intracellular environments. During adaptation to intracellular life, microbes often exhibit a significant reduction in their genome size, resulting in the loss of metabolic and structural elements that are not required for life within a host cell. The bacterial cell wall, composed of peptidoglycan (PG), protects most bacterial species from osmotic stress and is essential for cell division. PG also determines a bacterial cell’s shape, and by directing its synthesis and degradation microbes can effectively control cell size and differentiation between developmental forms. Nascent PG biosynthesis is spatially and temporally restricted within bacterial cells via two known molecular complexes: the MreB complex, which is primarily associated with bacterial cell wall synthesis, and the FtsZ complex, which is associated with septal PG synthesis required during cell division. Members of the Chlamydiae do not encode FtsZ and had long been thought to completely lack PG. We have found that s everal members of the Chlamydiaceae do in fact synthesize PG but do not use it to form a canonical cell wall. Instead, these microbes utilize only septal PG in their replicative forms, which is maintained, paradoxically, by an MreB complex. Our most recent findings indicate that this has likely happened at least two separate times during the evolution of these microbes, as a member of the Simkaniaceae similarly lacks a sacculus but retains septal PG. Here we propose a series of studies to continue our investigation of how members of the Chlamydiales temporally and spatially restrict PG synthesis throughout the division process, efficiently controlling cell size, division, and the transition between developmental forms. Our major goals for the next five years are: 1) to continue to characterize the molecular mechanisms by which these organisms direct peptidoglycan synthesis and degradation in the absence of FtsZ, 2) to investigate polar localizing features present in Chlamydia and assess their role in chromosomal partitioning and the process of orienting the peptidoglycan ‘ring’ and the cell division complex, and 3) to examine components of the chlamydial outer membrane essential for enabling the processes of division and bacterial cell size reduction that must occur prior to developmental form conversion. Genetically reduced microbes are attractive models for identifying the fundamental components of essential physiological processes. These planned studies will elucidate not only how genetically reduced microbes regulate cell size and divide in osmotically stable environments, but also illuminate the inherent versatility of the broadly conserved molecular complexes underlying these processes.