xln23 from Streptomyces chattanoogensis UAH23 encodes a putative enzyme with separate xylanase and arabinofuranosidase catalytic domains

The xylanase gene xysA of Streptomyces halstedii JM8 was used to isolate a DNA fragment from a gene library of Psti-digested chromosomal DNA of the lignocellulolytic actinomycete Streptomyces chattanoogensis CECT-3336. Nucleotide sequence analysis revealed a gene (xln23) encoding a bifunctional multi-modular enzyme bearing two independent xylanase and alpha -L-arabinofuranosidase domains separated by a Ser/Gly-rich linker. The N terminus of the predicted protein showed high homology to family F xylanases. The C terminus was homologous to amino acid sequences found in enzymes included in the glycosyl hydrolase family 62 and, in particular, to those of alpha -L-arabinofuranosidase AbsB from Streptomyces lividans. PCR and RT-PCR experiments showed that the nucleotide sequences corresponding to each domain are arranged as expected on the chromosomal DNA and that they are cotranscribed. To our knowledge, this is the first description of xylanase and arabino-furanosidase domains in a same open reading frame.