Functions of the prp operon in carbon source metabolism and acetic acid resistance in acetic acid bacteria: a case study in Acetobacter pasteurianus CGMCC 1.41.

BACKGROUND: prp operon usually comprises prpB, prpC, prpD and prpE genes, encoding 2-methylisocitrate (2-MIC) lyase, 2-methylcitrate (2-MC) synthase, 2-MC dehydratase and propionyl-CoA synthetase, respectively, which constitute the pathway 2-methylcitrate cycle (2-MCC) well-known for microbial propionate metabolism. Acetic acid bacteria (AAB) represent a group of obligate aerobic and Gram-negative microorganisms. Attributed to the robust ethanol-oxidating and acetic acid-resisting abilities, vinegar production stands for an important AAB's industrial application. Previously, we unveiled the first AAB's prp operon in Acetobacter pasteurianus CGMCC 1.41-a vinegar-brewing strain, and its up-regulation during acetic acid fermentation, suggesting the genes' potential contribution to the adaptive processes of this strain. In this study, the prevalence of prp genes among AAB were analyzed, followed by constructing six markerless deletion mutants to investigate the genes' role in carbon source metabolism and acetic acid resistance in A. pasteurianus CGMCC 1.41.

RESULTS: The prp gene cluster lacking prpE was found widespread in the genus Acetobacter. A gene encoding PrpE-homologous acetyl-CoA synthetase, designated prpE', was discovered to participate the formation of the putative operon in many AAB genomes. Growth experiments revealed the ability to utilize propionate as a carbon source of A. pasteurianus CGMCC 1.41, which simultaneously required the function of PrpB, PrpC and PrpD. A role of PrpD or PrpB in promoting ethanol assimilation was observed, whereas PrpE' appeared to be important for metabolizing both ethanol and acetic acid as carbon sources. PrpB, PrpC and PrpD also exerted a positive effect on glycerol metabolism. Moreover, the integrity of the 2-MCC pathway exhibited a great importance upon the initiation of the bioreaction catalyzed by PrpC. Finally, PrpE' was assumed to be critical for an acetic acid-resisting process-acetic acid overoxidation, which might be potentially enhanced by PrpD as well.

CONCLUSION: The present work expands the knowledge to the prp operon in the context of AAB, which also indicates the significance of gaining deeper insight into the specific roles of relevant genes in this group of bacteria.