Until a few years ago the description of the pathways of transport and metabolism of galactosides in lactic acid bacteria was mainly phenomenological.Different transport and enzymatic activities had been detected but the individual components (proteins, enzymes) were largely unknown and the genes had not yet been isolated.In this paper 1will address the different transport mechanisms and metabolic pathways of galactosides in factic acid bacteria, and indicate the meleeular level at which these studies have evolved.The genes encoding galactosidetransport ATPases ion Iinked galactoside transporters and phosphoenolpyruvate:galactoside phosphotransferase systems (PTS) have been isolated and characterized from a number of lactic acid bacteria.In addition, the genes encoding subsequent steps of lactose (galactose) metabolism, ie those encoding the Leloir and Tagatose 6 phosphate pathway enzymes, have been characterized at a genetic and to some extent at a biochemical javel.The genes encoding the transport proteins and metabolic enzymes are often clustered in a single operon or regulon which allows co-ordinate expression of the polypeptides.Transport of galactosides has most been thoroughly studied in Streptococcus thermophi/us where the transport gene encodes a chimeric protein (LacS) that is composed of a carrier domain homofogous to various ion Iinked carbohydrate transporters and a hydrophilic domain hornologous to the liA phosphoryl transfer protein of various PTSs.LacS has been extracted from membrane vesicles and functionally reconstituted into proteoliposomes, and the energetics and kinetic mechanism of transport have been analyzed.lacS mediates galactoside H+ symport and galactoside exchange (eg lactose/galactose exchange) of which the latter reaction is highly favoured under physiological conditions.Details of the galactoside translocation pathway have been estab-Iished by studying mutant LacS proteins in which the galactoside movement is uncoupled from the H+ transport whereas the exchange reaction takes place normally.The data offer a rationale for the observed excretion of galactose by Gal-and Galt strains of S thermophilus under conditions of growth in the presence of excess lactose. sugar transport ATPase / ion-Iinked transport / exchange / PTS / galactoside metabolismRésumé -Biochimie et biologie moléculaire du transport et du métabolisme des galactosides chez les bactéries lactiques.1/ y a quelques années encore, la description des voies de transport et du métabolisme des galactosides chez les bactéries lactiques était principalement phénoménologique.Des systèmes de transport et des activités enzymatiques différentes avaient été détectés, mais leurs composants individuels (protéines, enzymes) étaient pour la plupart inconnus et des ions H+ alors que la réaction d'échange se déroule normalement.Les données offrent une explication pour l'excrétion de galactose observée dans les souches GaJ+ et eer de S thermophilus dans des conditions de croissance en présence d'un excès de lactose. ATPase du transport des glucides / transport ions-dépendant / échange / PTS / métabolisme des galactosides
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B. Poolman (1993) studied this question.