Distribution and evolution of the xylanase genes xynA and xynB and their homologues in strains of Butyrivibrio fibrisolvens

The ruminal bacterium Butyrivibrio fibrisolvens is being engineered by the introduction of heterologous xylanase genes in an attempt to improve the ut ilization of plant material in ruminants. However, relatively little is kno wn about the diversity and distribution of the native xylanase genes in str ains of B. fibrisolvens. In order to identify the most appropriate hosts fo r such modifications, the xylanase genotypes of 28 strains from the three 1 6S ribosomal DNA (rDNA) subgroups of Butyrivibrio fibrisolvens have been in vestigated, Only 4 of the 20 strains from 16S rDNA group 2 contained homolo gues of the strain Bu49 xynA gene. However, these four xynA-containing stra ins, and two other group 2 strains, contained members of a second xylanase gene family clearly related to xynA (subfamily I). Homologues of xynB, a se cond previously described xylanase gene from B. fibrisolvens, were identifi ed only in three of the seven group 1 strains and not in the group 2 and 3 strains. However, six of the group 1 strains contained one or more members of the two subfamilies of homologues of xynA. The distribution of genes and the nucleotide sequence relationships between the members of the two xynA subfamilies are consistent with the progenitor of all strains of B. fibriso lvens having contained a xynA subfamily I gene. Since many xylanolytic stra ins of B. fibrisolvens did not contain members of either of the xynA subfam ilies or of the xynB family, at least one additional xylanase gene family r emains to be identified in B. fibrisolvens.