Dual role of alpha-acetolactate decarboxylase in Lactococcus lactis subsp. lactis

Abstract

The ␣-acetolactate decarboxylase gene aldB is clustered with the genes for the branched-chain amino acids (BCAA) in Lactococcus lactis subsp. lactis. It can be transcribed with BCAA genes under isoleucine regulation or independently of BCAA synthesis under the control of its own promoter. The product of aldB is responsible for leucine sensibility under valine starvation. In the presence of more than 10 M leucine, the ␣-acetolactate produced by the biosynthetic acetohydroxy acid synthase IlvBN is transformed to acetoin by AldB and, consequently, is not available for valine synthesis. AldB is also involved in acetoin formation in the 2,3-butanediol pathway, initiated by the catabolic acetolactate synthase, AlsS. The differences in the genetic organization, the expression, and the kinetics parameters of these enzymes between L. lactis and Klebsiella terrigena, Bacillus subtilis, or Leuconostoc oenos suggest that this pathway plays a different role in the metabolism in these bacteria. Thus, the ␣-acetolactate decarboxylase from L. lactis plays a dual role in the cell: (i) as key regulator of valine and leucine biosynthesis, by controlling the acetolactate flux by a shift to catabolism; and (ii) as an enzyme catalyzing the second step of the 2,3-butanediol pathway.Synthesis of the three branched-chain amino acids (BCAA), leucine, isoleucine, and valine, has been studied in detail in organisms as diverse as bacteria, fungi, and plants (for reviews, see references 8, 32, 57, and 58). A particular feature of this synthesis is that it is carried out, in part, by the same enzymes for the three amino acids (Fig. 1). However, the relative amounts of the three amino acids in cell proteins are not the same, since leucine, valine, and isoleucine represent 39, 36, and 25%, respectively, of the total BCAA in Escherichia coli (40). Regulation of the BCAA synthesis is therefore necessarily complex. A global regulation model has been established only for E. coli (58). Two levels of regulation, concerning gene transcription and enzyme inhibition, have previously been described.E. coli genes which encode BCAA synthesis enzymes form three clusters and are organized in five transcription units (2). Initiation of transcription of all the genes except ilvC is regulated by nonspecific, pleiotropic regulators responding to amino acid starvation and different changes in the medium (58). In addition to this control, a mechanism of transcriptional attenuation dependent on the synthesis of a leader peptide as described first for the tryptophan operon (33) negatively regulates ilvBN, ilvGMEDA, and leuABCD. These mechanisms allow the expression of the enzymes necessary for BCAA synthesis when needed.Activity of several BCAA-synthesizing enzymes is regulated by retroinhibition (Fig. 1). This control is simple in the case of leucine and isoleucine, since each amino acid inhibits an enzyme specific for an early step of its synthesis (LeuA and IlvA, respectively [53,57]). In contrast, the first enzyme involved in valine synthesis is also required fo...

Overall control of nitrogen metabolism in Lactococcus lactis by CodY, and possible models for CodY regulation in Firmicutes

“…Since the CodY signal in L. lactis is essentially mediated by isoleucine, excess of this amino acid in the environment leads to growth inhibition by blocking inappropriately CodY-dependent biosynthesis pathways. Several studies in chemically defined media have reported a CodY-dependent isoleucine growth inihibition in L. lactis linked to repression of the biosynthesis of valine and leucine (Goupil-Feuillerat et al, 1997), asparagine, histidine, lysine and threonine (this work) and glutamate (Lapujade et al, 1998). In the latter case, the growth defect was shown to be the result of a reduced synthesis of 2-oxoglutarate associated with a low level of the glutamate synthase (Lapujade et al, 1998).…”

The Lactococcus lactis CodY Regulon

“…As we show that this operon is one of major targets of CodY (TABLE THREE) and CodYboxes are present in its upstream and coding regions (TABLE FOUR), CodY most probably directly controls ilv expression. Moreover, repression of this operon has been shown to be dependent on the BCAA Ile (51)(52)(53), which is the most potent modulator of CodY activity, as it has been shown that CodY binding to the promoter region of at least one of its targets, oppD, in DNA binding studies is stimulated most by this particular BCAA (17).…”

“…A different group of genes, the transcription of which was found to be elevated in L. lactis MG1363⌬codY, encompasses those involved in BCAA metabolism; the ilv-ald operon for BCAA biosynthesis (51,52) was derepressed over five times. Thus, although L. lactis MG1363 like many other lactococcal dairy strains is auxotrophic for these amino acids, several genes of the BCAA biosynthetic operon are present on the chromosome and are actively transcribed (1,53).…”

Selection and properties of α-acetolactate decarboxylase-deficient spontaneous mutants of Streptococcus thermophilus

“…Such strains are widely used in butter-making processes (Veringa et al, 1976) and for 58 the production of aroma additives with a high -acetolactate (Kuiper et al, 1987) or diacetyl 59 (Jönsson et al, 1980) content. In Lactococcus lactis strains isolated from non-dairy 60 environments, -acetolactate decarboxylase is also involved in the regulation of leucine and 61 valine biosynthesis (Goupil-Feuillerat et al, 1997). Indeed, -acetolactate is a precursor of 62 these branched-chain amino acids, and when they are present in excess, the flux of -63 acetolactate is diverted towards acetoin, due to an allosteric activation of the -acetolactate 64 decarboxylase (Phalip et al, 1994) and to a translational regulation of -acetolactate g/l), L-aspartate (300 mg/l) and L-tyrosine (100 mg/l).…”