Transcriptional analysis of levU operon encoding saccharolytic enzymes and two apparent genes involved in amino acid biosynthesis in Zymomonas mobilis

The bacteriophage-derived transcriptional regulator, LscR, activates the expression of levansucrase genes in Pseudomonas syringae

Synthesis of the exopolysaccharide levan occurs in the bacterial blight pathogen of soybean, Pseudomonas syringae pv. glycinea PG4180, when this bacterium encounters moderate to high concentrations of sucrose inside its host plant. The process is mediated by the temperature-dependent expression and secretion of two levansucrases, LscB and LscC. Previous studies showed the importance of a prophage-associated promoter element in driving the expression of levansucrase genes. Herein, heterologous screening for transcriptional activators revealed that the prophage-borne transcriptional regulator, LscR, from P. syringae mediates expression of levansucrase. A lscR-deficient mutant was generated and exhibited a levan-negative phenotype when grown on a sucrose-rich medium. This phenotype was confirmed by zymographic analysis and Western blots which demonstrated absence of levansucrase in the supernatant and total cell lysates. Transcriptional analysis showed a down-regulation of expression levels of levansucrase and glycosyl hydrolase genes in the lscR-deficient mutant. Ultimately, a direct binding of LscR to the promoter region of levansucrase was demonstrated using electrophoretic mobility shift assays allowing to conclude that a bacteriophage-derived regulator dictates expression of bacterial genes involved in in planta fitness.

Zymomonas mobilis for fuel ethanol and higher value products

High oil prices, increasing focus on renewable carbohydrate-based feedstocks for fuels and chemicals, and the recent publication of its genome sequence, have provided continuing stimulus for studies on Zymomonas mobilis. However, despite its apparent advantages of higher yields and faster specific rates when compared to yeasts, no commercial scale fermentations currently exist which use Z. mobilis for the manufacture of fuel ethanol. This may change with the recent announcement of a Dupont/Broin partnership to develop a process for conversion of lignocellulosic residues, such as corn stover, to fuel ethanol using recombinant strains of Z. mobilis. The research leading to the construction of these strains, and their fermentation characteristics, are described in the present review. The review also addresses opportunities offered by Z. mobilis for higher value products through its metabolic engineering and use of specific high activity enzymes.

Disruption of the Zymomonas mobilis extracellular sucrase gene (sacC) improves levan production

AIMS

Disruption of the extracellular Zymomonas mobilis sucrase gene (sacC) to improve levan production.

METHODS AND RESULTS

A PCR-amplified tetracycline resistance cassette was inserted within the cloned sacC gene in pZS2811. The recombinant construct was transferred to Z. mobilis by electroporation. The Z. mobilis sacC gene, encoding an efficient extracellular sucrase, was inactivated. A sacC defective mutant of Z. mobilis, which resulted from homologous recombination, was selected and the sacC gene disruption was confirmed by PCR. Fermentation trials with this mutant were conducted, and levansucrase activity and levan production were measured. In sucrose medium, the sacC mutant strain produced threefold higher levansucrase (SacB) than the parent strain. This resulted in higher levels of levan production, whilst ethanol production was considerably decreased.

CONCLUSIONS

Zymomonas mobilis sacC gene encoding an extracellular sucrase was inactivated by gene disruption. This sacC mutant strain produced higher level of levan in sucrose medium because of the improved levansucrase (SacB) than the parent strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

The Z. mobilis CT2, sacC mutant produces high level of levansucrase (SacB) and can be used for the production of levan.

yveB, Encoding endolevanase LevB, is part of the sacB-yveB-yveA levansucrase tricistronic operon in Bacillus subtilis

Transcription of sacB, yveB and yveA, three clustered genes on the Bacillus subtilis chromosome, is simultaneously induced by sucrose. Northern blotting analyses with specific probes showed three distinct mRNAs: a monocistronic 1.7 kb sacB mRNA, a bicistronic 3.3 kb sacB-yveB mRNA and a tricistronic 4.9 kb sacB-yveB-yveA mRNA. These results indicate that sacB, encoding levansucrase, is the proximal gene of a sucrose-inducible operon that includes the two other genes. The yield of the full-length transcript is lower than that of the bicistronic transcript, whose yield is itself lower than that of the monocistronic transcript. This suggested that the 3' terminal parts of sacB and yveB genes worked as internal terminator structures. The protein encoded by yveB, which remains anchored to the membrane, displays an endolevanase activity, which, coupled with exolevanase activity of SacB, leads to a complete degradation of levan, a branched fructosyl polymer. It is proposed to rename yveB as levB.