A BIFUNCTIONAL ENZYME, WITH SEPARATE XYLANASE AND BETA(1,3-1,4)-GLUCANASE DOMAINS, ENCODED BY THE XYND GENE OF RUMINOCOCCUS-FLAVEFACIENS

Adjacent regions of a Ruminococcus flavefaciens 17 DNA fragment were f ound to encode xylanase and beta(1,3-1,4)-glucanase activities. Sequen cing of this fragment showed that both activities are encoded by a sin gle 2,406-bp open reading frame corresponding to the xynD gene. The pr edicted product has a characteristic signal sequence that is followed by an amino-terminal domain related to family G xylanases, while the c arboxy-terminal domain is related to beta(1,3-1,4)-glucanases from sev eral other bacterial species. These two domains are connected by a reg ion of unknown function that consists of 309 amino acids and includes a 30-amino-acid threonine-rich sequence. A polypeptide having a molecu lar weight of approximately 90,000 and exhibiting xylanase and beta(1, 3-1,4)-glucanase activities was detected in Escherichia coli cells car rying the cloned xynD gene. This is one of the first cases in which a microbial polysaccharidase has been shown to carry separate catalytic domains active against different plant cell wall polysaccharides withi n the same polypeptide. xynD is one of a family of related genes in R. flavefaciens that encode enzymes having multiple catalytic domains, a nd the amino terminus of XYLD exhibits a high degree of similarity wit h the corresponding regions of another xylanase, XYLA, which carries t wo different xylanase catalytic domains.