Through screening of a Zymomonas mutant library the himA gene was found to be involved in the inhibitory effect of acetate on Zymomonas performance. Xylose-utilizing Zymomonas strains further engineered to reduce activity of the himA gene were found to have increased ethanol production in comparison to a parental strain, when cultured in mixed-sugars medium comprising xylose, and, in particular, in the presence of acetate.
1. A process for producing ethanol from a mixed-sugars medium comprising xylose, comprising:
(a) providing a recombinant Zymomonas strain capable of utilizing xylose to produce ethanol, said strain comprising at least one genetic modification which reduces expression of integration host factor alpha subunit protein (HimA); and
(b) culturing the strain of (a) in a medium comprising xylose, whereby xylose is used by said strain as a carbon source for producing ethanol.
2. The process according to claim 1 , wherein culturing is at a temperature range of from about 25° C. to about 40° C. in medium with pH in a range that is between about 4.5 and 7.5.
3. The process according to claim 2 , wherein the temperature is between about 30° C. and 37° C.
4. The process according to claim 2 , wherein the pH is between about 5.8 and 7.0.
5. The process according to claim 1 , wherein the genetic modification is a modification of the himA gene selected from the group consisting of insertion, deletion, mutation, cosuppression, and antisense RNA expression.
6. The process according to claim 5 , wherein the genetic modification makes the himA gene non-functional.
7. The process according to claim 6 , wherein the genetic modification is an insertion introduced into the himA gene by homologous recombination.
8. The process according to claim 1 , wherein the recombinant Zymomonas strain is ZW801-4:: ΔhimA or strain AcR#3 which are derived from strain ATCC PTA 7858.
9. The process according to claim 1 , wherein the medium further comprises acetate and wherein the recombinant Zymomonas strain produces an increased amount of ethanol as compared to its parental Zymomonas strain with no genetic modification which reduces himA expression.
10. The process of claim 9 , wherein said increased amount of ethanol produced is due to acetate resistance of the Zymomonas strain.
11. The process of claim 9 , wherein the medium comprises at least about 120 g/L of mixed sugars comprising xylose, a sugar alcohol selected from the group consisting of sorbitol, mannitol, galactitol, ribitol, and a mixture thereof in a final concentration that is between about 2 mM and about 100 mM, and at least about 6 g/L acetate.
12. The process of claim 11 , wherein the sugar alcohol is in a concentration that is between about 5 mM and about 20 mM.