Zymomonas mobilis IFO 13 756 could take up galactose and metabolize it so that galactonic acid accumulated in the culture fluid. The oxidation of galactose was catalysed by a membrane-associated enzyme, which could be solubilized with a mixture of 1% Triton X-100 and 1 M KC1. The strain also accumulated galactitol as a minor metabolite. To confer the ability to utilize galactose on Z. mobilis, a Gal + recombinant plasmid, pZG13, was constructed by the insertion of the galETK genes of Escherichia coli immediately downstream of the Z. mobilis promoter gene in pZA22, the plasmid introduced into a Zymomonas strain. Uridine diphospho-glucose 4-epimerase coded in the galE gene was expressed in Z. mobilis carrying pTG13. The recombinant strain could produce a small amount of ethanol from galactose. results were reported by Buchholz et al. (1989) and also by G o o d m a n et al. (1984). It is not clear why Zymomonas does not grow on lactose. One possible answer to this question may be growth inhibition by galactose or its metabolites (Yanase et al. 1988). In enteric bacteria, galactose is converted to glucose-6-phosphate via the LeLoir pathway, in which uridine diphospho(UDP)-glucose 4-epimerase (coded by the galE gene), galactose-l-phosphate uridylyltransferase (coded by the gaiT gene), galactokinase (coded by the galK gene), and phosphoglucomutase take part (Shapiro and Adhya 1969).
So far nothing is known about galactose metabolism in Zymomonas. Here, we have studied the metabolism of galactose in Z. mobilis using a wild-type strain and a recombinant strain containing the E. coli galactose operon.