Over‐expression of xylulokinase in a xylose‐metabolising recombinant strain of Zymomonas mobilis

The broad host range vector pBBR1MCS‐2 has been evaluated as an expression vector for Zymomonas mobilis . The transformation efficiency of this vector was 2 × 10 3 CFU per μg of DNA in a recombinant strain of Z. mobilis ZM4/Ac R containing the plasmid pZB5. Stable replication for this expression vector was demonstrated for 50 generations. This vector was used to study xylose metabolism in acetate resistant Z. mobilis ZM4/Ac R (pZB5) by over‐expression of xylulokinase (XK), as previous studies had suggested that XK could be the rate‐limiting enzyme for such strains. Based on the above vector, a recombinant plasmid pJX1 harboring xyl B (expressing XK) under control of a native Z. mobilis promotor P pdc was constructed. When this plasmid was introduced into ZM4/Ac R (pZB5) a 3‐fold higher XK expression was found compared to the control strain. However, fermentation studies with ZM4/Ac R (pZB5, pJX1) on xylose medium did not result in any increase in rate of growth or xylose metabolism, suggesting that XK expression was not rate‐limiting for ZM4/Ac R (pZB5) and related strains.