Organisation and Variable Incidence of Genes Concerned with the Utilization of Xylans in the Rumen Cellulolytic Bacterium Ruminococcus flavefaciens

Strains of the rumen cellulolytic bacterium Ruminococcus flavefaciens vary in their ability to utilise isolated plant xylans for growth. Here an 11.5 kb fragment of genomic DNA from the xylan-utilizing R. flavefaciens strain 17 that contains the xynD gene, which encodes an extracellular xylanase/β -(1,3-1,4)-glucanase, was analysed. Sequencing revealed five consecutive open reading frames downstream from xynD on the same strand, preceded by the divergently transcribed ORF3. These include the following genes likely to be involved in utilisation of xylan breakdown products: xylA, encoding a β -(1,4)-xylosidase, xsi, encoding a xylose isomerase and ORF8 encoding part of an ABC-type sugar transporter. The products of ORF3 and of a partial ORF1 found upstream of xynD, show significant sequence similarity to AraC-type regulatory proteins while ORF4 and ORF7 show no close relationship to other known proteins. Homologues of the xylA and xsi genes, and inducible β -xylosidase activity, were readily detectable in three xylan-utilizing R. flavefaciens strains 17, B1a and C94 but not in two xylan non-utilizing strains, C1a and B34b, suggesting that this cluster may be absent from xylan non-utilizing strains.