Regulation of branched-chain amino acid biosynthesis by alpha-acetolactate decarboxylase in Streptococcus thermophilus.
OBJECTIVE
To demonstrate the presence of an active alpha-acetolactate decarboxylase in Streptococcus thermophilus and to investigate its physiological function.


RESULTS
Streptococcus thermophilus CNRZ385 contains a gene encoding an alpha-acetolactate decarboxylase. Comparison of the production of alpha-acetolactate and its decarboxylation products, by the parent strain and an alpha-acetolactate decarboxylase-deficient mutant, demonstrated the presence of a control of the pool of alpha-acetolactate by valine, leucine and isoleucine. This control occurs via an allosteric activation of the alpha-acetolactate decarboxylase. Cell-free extracts of S. thermophilus were not able to decarboxylate the isoleucine precursor alpha-acetohydroxybutyrate.


CONCLUSIONS
These results strongly suggest that one of the physiological functions of the alpha-acetolactate decarboxylase in S. thermophilus is to regulate leucine and valine biosynthesis by diverting the flux of alpha-acetolactate towards acetoin when the branched-chain amino acids are present at a high concentration.


CONCLUSIONS
Regulation of branched-chain amino acid biosynthesis by alpha-acetolactate decarboxylase may occur in several other micro-organisms and explain some of their growth properties.
Monnet
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C
Unité Mixte de Recherche Génie et Microbiologie des Procés Alimentaires, Institut National de la Recherche Agronomique, Thiverval-Grignon, France. monnet@grignon.inra.fr
Nardi
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M
Hols
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P
Gulea
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Corrieu
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Monnet
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V
D000093:Acetoin / Q000378:metabolism; D000597:Amino Acids, Branched-Chain / Q000096:biosynthesis*; D002262:Carboxy-Lyases / Q000235:genetics / Q000378:metabolism*; D003470:Culture Media; D003931:Diacetyl / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D006885:Hydroxybutyrates / Q000378:metabolism; D007773:Lactates / Q000378:metabolism; D009154:Mutation; D013291:Streptococcus / Q000201:enzymology* / Q000235:genetics
D000597:Amino Acids, Branched-Chain; D003470:Culture Media; D006885:Hydroxybutyrates; D007773:Lactates; C027745:alpha-aceto-alpha-hydroxybutyrate; C006359:alpha-acetolactate; D000093:Acetoin; D002262:Carboxy-Lyases; C023746:acetolactate decarboxylase; D003931:Diacetyl