Abstract
The rod-shaped cells of Myxococcus xanthus, a Gram-negative deltaproteobacterium, differentiate to environmentally resistant spores upon starvation or chemical stress. The environmental resistance depends on a spore coat polysaccharide that is synthesised by the ExoA-I proteins, some of which are part of a Wzx/Wzy-dependent pathway for polysaccharide synthesis and export; however, key components of this pathway have remained unidentified. Here, we identify and characterise two additional loci encoding proteins with homology to enzymes involved in polysaccharide synthesis and export, as well as sugar modification and show that six of the proteins encoded by these loci are essential for the formation of environmentally resistant spores. Our data support that MXAN_3260, renamed ExoM and MXAN_3026, renamed ExoJ, are the Wzx flippase and Wzy polymerase, respectively, responsible for translocation and polymerisation of the repeat unit of the spore coat polysaccharide. Moreover, we provide evidence that three glycosyltransferases (MXAN_3027/ExoK, MXAN_3262/ExoO and MXAN_3263/ExoP) and a polysaccharide deacetylase (MXAN_3259/ExoL) are important for formation of the intact spore coat, while ExoE is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for initiating repeat unit synthesis, likely by transferring N-acetylgalactosamine-1-P to undecaprenyl-phosphate. Together, our data generate a more complete model of the Exo pathway for spore coat polysaccharide biosynthesis and export.
Highlights
Bacteria have evolved different strategies that enable their survival in response to environmental changes
The environmental resistance depends on a spore coat polysaccharide that is synthesized by the ExoA-I proteins, some of which are part of a Wzx/Wzy-dependent pathway for polysaccharide synthesis and export; key components of this pathway have remained unidentified
We identify and characterize two additional loci encoding proteins with homology to enzymes involved in polysaccharide synthesis and export, as well as sugar modification, and show that six of the proteins encoded by these loci are essential for the formation of environmentally resistant spores
Summary
Bacteria have evolved different strategies that enable their survival in response to environmental changes Often these strategies include alterations in gene expression, motility behaviour and/or metabolism without evident changes in cell morphology. The best-studied examples of environmentally induced bacterial differentiation include spore formation in three phylogenetically widely separated species Bacillus subtilis (Tan & Ramamurthi, 2014), Streptomyces coelicolor (Flärdh & Buttner, 2009), and Myxococcus xanthus (Konovalova et al, 2010). In the Gram-negative deltaproteobacterium M. xanthus, sporulation is typically induced by starvation (Konovalova et al, 2010) In this bacterium, spores are formed independently of a cell division event, and during the sporulation process, PG is replaced by a spore coat consisting mainly of polysaccharide. We focus on the identification of proteins important for formation of the spore coat polysaccharide in M. xanthus
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