The first attempt to introduce an L-arabinose utilization pathway in S. cere­visiae by heterologous expression of the complete E. coli L-arabinose pathway
did not result in appreciable arabinose utilization [70], most likely due to the absence of functional expression of the L-arabinose isomerase. It was only when the E. coli araA gene encoding the L-arabinose isomerase was substi­tuted by the corresponding Bacillus subtilis gene that a functional arabinose pathway was established in S. cerevisiae [71]. Similar to the use of the het­erologous XI pathway, other genetic modifications in addition to the new L-arabinose isomerase were required for the recombinant strain to grow on L-arabinose as sole carbon source [71]: an additional copy of the galactose permease (Gal2), which also transports arabinose [72], and an unspecified adaptation for growth on arabinose [71].

The fungal L-arabinose utilization pathway has also been introduced in S. cerevisiae, combining enzymes from P. stipitis and from the filamentous fungus Trichoderma reesei. The enzymes were actively expressed; however, neither appreciable growth on L-arabinose nor significant ethanolic fermen­tation was observed [73]. The dysfunction of the fungal arabinose pathway with respect to ethanolic fermentation parallels the inability of the naturally arabinose-growing yeasts to ferment L-arabinose to ethanol [50,69]. Instead, these yeasts often produce L-arabitol from L-arabinose (Fig. 2) [65,66,69]. Minute ethanolic fermentation has been observed for six yeast species, C. arabinofermentans, P. guilliermondii, C. auringiensis, C. succiphila, Ambro- siozyma monospora, and Candida sp. YB-2248, but only in rich medium [65, 69]. Rich media may contain other fermentable sugars as well as undefined electron acceptors that serve to regenerate reduced cofactors [32,74-76], which appears necessary for ethanolic arabinose fermentation to occur via the fungal pathway. Also, the presence of low amounts of oxygen aids cofactor regeneration [50,77].

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