Arabinan is a polysaccharide found in plant cell walls as a side chain of pectin. H. jecorina is also able to degrade arabinan to L-arabinose by an arabinanolytic system. To date two arabinanases have been characterized: an α -L-arabinofuranosidases (ABF1) and a íŸ-xylosidase which has a separate α arabinofuranosidase domain and activity. L-arabinose can be used by the fungus as a carbon source via the L-arabinose catabolic pathway, which converts L-arabinose to D-xylulose 5-phosphate. Little is known about the regulation of the arabinanase system in H. jecorina, however. The Zn(II)-Cys6-protein XYR1/XlnR has been shown to be a transcriptional activator of cellulase and xylanase gene transcription in different Aspergillus spp. and in H. jecorina. XlnR does not regulate arabinan and L-arabinose metabolism in the Aspergilli. In H. jecorina, however, the aldose reductase XYL1 (which is also involved in L-arabinose catabolism) is controlled by XYR1, thus implying that XYR1 would also regulate L-arabinose metabolism. However, Stricker et al. (2006) reported that a knock out in xyr1 in H. jecorina did not affect growth on D-arabinose. These conflicting data prompted us to perform a more detailed investigation on the possibility of arabinan and L-arabinose metabolism by XYR1. In this paper, we will provide evidence that XYR1 is indeed essential for expression of the enzymes of arabinan and D-arabinose degradation. Consistent with a role of L-arabinitol as inducer of the arabinolytic system, L-arabinitol dehydrogenase-knock out mutants display strongly increased levels of arabinofuranosidase gene expression. However, L-arabinitol cannot rescue gene expression in delta-xyr1 and delta-xyl1 strains, indicating a cross-talk of XYR1, XYL1 and L-arabinitol in the induction of these genes.
Full conference title:
9th EUROPEAN CONFERENCE ON FUNGAL GENETICS
- ECFG 9th (2008)