Xylanase B and an arabinofuranosidase from Pseudomonas fluorescens subsp. cellulosa contain identical cellulose-binding domains and are encoded by adjacent genes

Abstract

The complete nucleotide sequence of the Pseudomonas fluorescens subsp. cellulosa xynB gene, encoding an endo-β-1,4-xylanase (xylanase B; XYLB) has been determined. The structural gene consists of an open reading frame (ORF) of 1775 bp coding for a protein of M(r) 61000. A second ORF (xynC) of 1712 bp, which starts 148 bp downstream of xynB, encodes a protein, designated xylanase C (XYLC), of M(r) 59000. XYLB hydrolyses oat spelt xylan to xylobiose and xylose, whereas XYLC releases only arabinose from the same substrate. Thus XYLB is a typical xylanase and XYLC is an arabinofuranosidase. Both enzymes bind to crystalline cellulose (Avicel), but not to xylan. The nucleotide sequences between residues 114 and 931 of xynB and xynC were identical, as were amino acid residues 39-311 of XYLB and XYLC. This conserved sequence is reiterated elsewhere in the P. fluorescens subsp. cellulosa genome. Truncated derivatives of XYLB and XYLC, in which the conserved sequence had been deleted, retained catalytic activity, but did not exhibit cellulose binding. A hybrid gene in which the 5' end of xynC, encoding residues 1-110 of XYLC, was fused to the Escherichia coli phoA' gene (encodes mature alkaline phosphatase) directed the synthesis of a fusion protein which exhibited alkaline phosphatase activity and bound to cellulose.