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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.
