Abstract
Sinorhizobium meliloti is an alphaproteobacterium belonging to the Rhizobiales Bacteria from this order elongate their cell wall at the new cell pole, generated by cell division. Screening for protein interaction partners of the previously characterized polar growth factors RgsP and RgsM, we identified the inner membrane components of the Tol-Pal system (TolQ and TolR) and novel Rgs (rhizobial growth and septation) proteins with unknown functions. TolQ, Pal, and all Rgs proteins, except for RgsE, were indispensable for S. meliloti cell growth. Six of the Rgs proteins, TolQ, and Pal localized to the growing cell pole in the cell elongation phase and to the septum in predivisional cells, and three Rgs proteins localized to the growing cell pole only. The putative FtsN-like protein RgsS contains a conserved SPOR domain and is indispensable at the early stages of cell division. The components of the Tol-Pal system were required at the late stages of cell division. RgsE, a homolog of the Agrobacterium tumefaciens growth pole ring protein GPR, has an important role in maintaining the normal growth rate and rod cell shape. RgsD is a periplasmic protein with the ability to bind peptidoglycan. Analysis of the phylogenetic distribution of the Rgs proteins showed that they are conserved in Rhizobiales and mostly absent from other alphaproteobacterial orders, suggesting a conserved role of these proteins in polar growth.IMPORTANCE Bacterial cell proliferation involves cell growth and septum formation followed by cell division. For cell growth, bacteria have evolved different complex mechanisms. The most prevalent growth mode of rod-shaped bacteria is cell elongation by incorporating new peptidoglycans in a dispersed manner along the sidewall. A small share of rod-shaped bacteria, including the alphaproteobacterial Rhizobiales, grow unipolarly. Here, we identified and initially characterized a set of Rgs (rhizobial growth and septation) proteins, which are involved in cell division and unipolar growth of Sinorhizobium meliloti and highly conserved in Rhizobiales Our data expand the knowledge of components of the polarly localized machinery driving cell wall growth and suggest a complex of Rgs proteins with components of the divisome, differing in composition between the polar cell elongation zone and the septum.
Highlights
The life cycle of unicellular bacteria includes genome replication and approximate duplication of cell size followed by cell division
We present nine further S. meliloti proteins with unknown functions that localize to sites of zonal PG synthesis, are required for cell growth, and involved in proteinprotein interactions with RgsP and RgsM
We further show that Rgs proteins interact with components of the Tol-Pal system, which is localized to sites of zonal PG synthesis and essential for S. meliloti cell division
Summary
The life cycle of unicellular bacteria includes genome replication and approximate duplication of cell size followed by cell division. Increasing the cell volume relies on cell wall growth, which requires elongation of the peptidoglycan (PG) sacculus. Most of the rod-shaped bacteria elongate by incorporating new PG in a dispersed manner along the sidewall, with the filaments of actin homolog MreB providing a scaffold for the elongasome PG biosynthesis machinery [1, 2]. The cell division process relies on constriction of the Z-ring that consists of the tubulin homolog FtsZ, a core component of the dynamic divisome complex [3]. Pal complex [4, 5] The latter consists of the inner membrane protein TolQ, the inner membraneanchored periplasmic proteins TolR and TolA, the outer membrane-anchored protein Pal and the
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