XYLANASE-B FROM NEOCALLIMASTIX PATRICIARUM CONTAINS A NONCATALYTIC 455-RESIDUE LINKER SEQUENCE COMPRISED OF 57 REPEATS OF AN OCTAPEPTIDE

A Neocallimastix patriciarum cDNA library was screened for xylanase-ex pressing clones, which were distinct from the previously characterized N. patriciarum xynA cDNA encoding xylanase A, A single cDNA, designat ed xynB, which did not exhibit homology with xynA, was isolated. North ern-blot analysis of mRNA from Avicel-grown N. patriciarum showed that xynB hybridized to a 3.4 kb mRNA species. The nucleotide sequence of xynB revealed a single open reading frame of 2580 bp coding for a prot ein designated xylanase B (XYLB), of M(r) 88066. The primary structure of XYLB was comprised of a 21-residue N-terminal signal peptide, foll owed by a 304-amino acid sequence that exhibited substantial homology with the catalytic domains of family F xylanases. The N-terminal domai n was linked to a C-terminal 70-residue sequence by a putative linker region, comprising 12 tandem repeats of a sequence containing TLPG as the core sequence, followed by an