12.3.1 Rumen fungi

Rumen anaerobic fungi, which are related to chytrids (66), have been shown to produce highly active cellulase systems (67) and have been the source of recombinant enzymes with high specific activities that have attracted interest for a variety of biotechnological applications [e. g. (68, 69)]. As in cellulolytic bacteria, individual enzyme structures show multidomain organization (70). Furthermore, manypolysaccharidases from anaerobic fungi exhibit a proposed 40 amino acid docking domain that maybe present in one, two, or three copies (71). This domain is cysteine-rich and shows no sequence homology with bacterial dockerins, but has been reported in enzymes from a range of anaerobic fungi including Orpinomyces, Piromyces, and Neocallimastix frontalis (72). There is evidence in Piromyces equi for an interaction between this docking domain and a scaffolding protein of 97 kDa (73). Recent evidence in Neocallimastix frontalis, however, also suggests that removal of the docking domain influences the activity and temperature optimum of the adjacent catalytic domains (74). The nature of the docking domain interaction with the putative scaffolding protein remains to be established.

The close sequence relationships between catalytic domains from cellulolytic anaerobic fungi and bacteria suggest that horizontal gene transfer between these two groups has played an important role in the evolution of their cellulase systems (75).