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J. Bacteriol. doi:10.1128/JB.00447-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Impact of aeration and heme-activated respiration on Lactococcus lactis gene expression: Identification of a heme-responsive operon

Department of Physiology, Fermentation Technology, and Department of Identification, Chr. Hansen A/S, DK-2970 Hørsholm, Denmark; Unité Bactéries Lactiques et Pathogènes Opportunistes, INRA UR888, Jouy en Josas, 78352, France

^* To whom correspondence should be addressed. Email: dkmbp{at}chr-hansen.com. alexandra.gruss{at}jouy.inra.fr.

	Abstract

Lactococcus lactis is a widely used food bacterium mainly characterized for its fermentation metabolism. However, this species undergoes a metabolic shift to respiration when heme is added to aerobic medium. Respiration results in markedly improved biomass and survival compared to fermentation. Whole genome microarrays were used to assess changes in L. lactis expression under aerobic and respiratory conditions, when compared to static growth, i.e. non-aerated. We observed that: i) Stress response genes were affected mainly by aerobic fermentation. This result underscores the differences between aerobic fermentation and respiration environments, and confirms that respiration growth alleviates oxidative stress. ii) Functions essential for respiratory metabolism e.g., genes encoding cytochrome bd oxidase, menaquinone biosynthesis, and heme uptake, are similarly expressed under the three conditions. This indicates that cells are prepared for respiration once O₂ and heme become available. iii) Expression of only 11 genes distinguishes respiration from both aerobic and static fermentation cultures. Among them, the genes comprising the putative ygfCBA operon are strongly induced by heme regardless of respiration, thus identifying the first heme-responsive operon in lactococci. We give experimental evidence that the ygfCBA genes are involved in heme homeostasis.