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Appl. Environ. Microbiol. doi:10.1128/AEM.01851-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Activation of diacetyl/acetoin pathway in Lactococcus lactis subsp. lactis biovar diacetylactis CRL264 by acidic growth

Departamento de Ciencias de Proteínas, Centro de Investigaciones Biológicas (CSIC). Ramiro de Maeztu 9, Madrid, España; Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET) and Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, (S2002LRK) Rosario, Argentina

	Abstract

Lactococcus lactis subsp. lactis biovar diacetylactis (L. diacetylactis) strains are aroma producing organisms used in starter cultures for the elaboration of dairy products. This species is essentially a fermentative microorganism, which co-metabolizes glucose and citrate to yield aroma compounds through the diacetyl/acetoin biosynthetic pathway. Our previous results have shown that under acidic growth L. diacetylactis CRL264 expresses coordinately the genes responsible for citrate transport and its conversion into pyruvate. In the present work the impact of acidic growth on glucose, citrate and pyruvate metabolism of L. diacetylactis CRL264 has been investigated by proteomic analysis. The results indicated that acid growth triggers conversion of citrate, but not glucose, into -acetolactate via pyruvate. Moreover, they showed that low pH has no influence on levels of lactate dehydrogenase and pyruvate dehydrogenase. Therefore, the influence of external pH on regulation of the diacetyl/acetoin biosynthetic pathway in L. diacetylactis CRL264 has been analyzed at the transcriptional level. Expression of the als, aldB, aldC and butBA genes encoding the enzymes involved in conversion of pyruvate into aromacompounds has been investigated by primer extension, RT-PCR analysis and transcriptional fusions. The results support that this biosynthetic pathway is induced at the transcriptional level by acidic growth conditions, presumably contributing to lactococcal pH homeostasis by synthesis of neutral compounds and by decreasing levels of pyruvate.