The scaffoldin protein of the cellulosome complex of the cellulolytic bacterium Clostridium thermocellum has nine cohesin modules to mount enzyme molecules onto the complex for efficient degradation of cellulosic mass. Although the scaffoldin bears only nine cohesins, it can express over 70 different dockerin-containing enzymes that can degrade diverse types of cellulosic materials into simpler soluble polysaccharides that the bacterium can use as energy source. It was shown recently that to determine which enzymes to express in response to the fluctuating composition of its extracellular substrate, C. thermocellum employs a transcription regulation system that involves alternative sigma-antisigma factors that resemble SigI-RsgI of Bacillus subtilis, which thus serves basis of our terminology. The C-termini of the anti-sigma factors (RsgIs) comprise protein modules which bind various carbohydrate materials and act as specific biosensors of specific components of biomass. We hypothesize that when a specific type of carbohydrate is bound to the C terminal module of an RsgI, a signal is transferred through its trans-membrane domain to release its cognate sigma factor, thereby enabling transcription. C. thermocellum possesses eight SigI-RsgI systems, which regulate genes related to plant cell wall degradation. Similar regulatory systems were found in several other cellulolytic bacteria using a bioinformatic approach. Here we describe the X-ray crystal structure determination of two C-terminal biomass sensors of the RsgI3 in C. thermocellum and Clostridium clariflavum, both of which are similar to a PA14 domain in the Bacillus anthracis protective antigen (PA). The structures were determined by molecular replacement using the structure of the B. anthracis PA14 domain. Both molecules form symmetric dimers by swapping domains, apparently imitating the architecture between the two similar tandem PA14 domains (PA14A and PA14B) that are present in both RsgI3’s and might play a role in carbohydrate binding.
