Acta Crystallographica Section D

Biological Crystallography

Volume 70, Part 2 (February 2014)


research papers



Acta Cryst. (2014). D70, 522-534    [ doi:10.1107/S139900471302926X ]

Fine-structural variance of family 3 carbohydrate-binding modules as extracellular biomass-sensing components of Clostridium thermocellum anti-[sigma]I factors

O. Yaniv, G. Fichman, I. Borovok, Y. Shoham, E. A. Bayer, R. Lamed, L. J. W. Shimon and F. Frolow

Abstract: The anaerobic, thermophilic, cellulosome-producing bacterium Clostridium thermocellum relies on a variety of carbohydrate-active enzymes in order to efficiently break down complex carbohydrates into utilizable simple sugars. The regulation mechanism of the cellulosomal genes was unknown until recently, when genomic analysis revealed a set of putative operons in C. thermocellum that encode [sigma]I factors (i.e. alternative [sigma] factors that control specialized regulon activation) and their cognate anti-[sigma]I factor (RsgI). These putative anti-[sigma]I-factor proteins have modules that are believed to be carbohydrate sensors. Three of these modules were crystallized and their three-dimensional structures were solved. The structures show a high overall degree of sequence and structural similarity to the cellulosomal family 3 carbohydrate-binding modules (CBM3s). The structures of the three carbohydrate sensors (RsgI-CBM3s) and a reference CBM3 are compared in the context of the structural determinants for the specificity of cellulose and complex carbohydrate binding. Fine structural variations among the RsgI-CBM3s appear to result in alternative substrate preferences for each of the sensors.

PDB references: 4b9c, 4b9p and 4b97

Keywords: Clostridium thermocellum; RsgI-CBM3; family 3 carbohydrate-binding modules; anti-[sigma]I factors.


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