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Submitted to: Current Microbiology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 9/23/2002 Publication Date: 7/1/2002 Citation: SKORY, C.D. INDUCTION OF RHIZOPUS ORYZAE PYRUVATE DECARBOXYLASE GENES. CURRENT MICROBIOLOGY. 2003. V. 47. P. 59-64. Interpretive Summary: Lactic acid has long been used by the food industry as an additive for preservation, flavor, and acidity. Recently, it has gained popularity for the manufacture of environmentally friendly products which include the biodegradable plastic, poly-lactic acid (PLA), and the chlorine-free solvent, ethyl lactate. The lactic acid market for the U.S. is in excess of 50,000 tons/year and could increase substantially if the market for PLA develops as expected. Lactic acid is typically made by microorganisms that are able to convert or ferment sugars obtained from agricultural crops, such as corn. Fermentations with the fungus Rhizopus are often preferred because the quality of the final product is superior to that obtained by bacterial fermentations. Furthermore, this organism produces lactic acid in the chemical form that is more desirable for making PLA. However, Rhizopus typically shifts to ethanol production with limited oxygen conditions, and this ultimately decreases the final yield of lactic acid. To better understand this regulation, we cloned the pyruvate decarboxylase genes from R. oryzae which are involved in ethanol formation and compared their expression levels during growth with varying oxygen availability. It may now be possible to use this information to modify the organism to redirect more of the cellular energy into production of lactic acid. Technical Abstract: Rhizopus oryzae can produce significant quantities of lactic acid when grown aerobically but shifts to ethanolic fermentation with anaerobically stressed conditions. To better understand this regulation, we cloned two pyruvate decarboxylase, pdcA and pdcB, genes from R. oryzae and compared expression levels during growth with varying oxygen availability. Both pdc ctranscripts and PDC (EC 4.1.1.1) activity were detected within 2 hours of transfer to a glucose containing medium but were approximately 10-fold higher for the oxygen limited culture. Over the next 4 hours, both transcripts continued to increase 2-3 fold under aerobic conditions. With limited oxygen availability, pdcA further increased 2-fold, while pdcB transcript levels decreased by approximately 20%. Total PDC activity more than doubled with oxygen limited growth. However, PDC activity under aerobic conditions was not detected after 6 hours, even though both transcript levels were increasing. |
