Em

lactic acid

Figure 2-13. Tagatose pathway in lactococci. Galactose-6-phos-phate is formed from hydrolysis of lactose-phosphate, the product of the lactose PTS. Isomerization and phosphorylation form tagatose-1,6-diphosphate, which is split by an aldolase, yielding the triose phosphates that feed into the EM pathway.

141 171 310 326 105 586 568 468

galactose-6-P tagatose-6-P tagatose-1,6- EIIAlac EIIBClac phospho-b -

isomerase kinase diP aldolase galactosidase

Figure 2-14. The lac operon in lactococci. The operon consists of four structural genes (lacABCD) coding for enzymes of the tagatose pathway, two genes, lacFE, coding for lactose-specific PTS proteins, and the lacG gene coding for phospho-p-galactosidase. A divergently transcribed lacR gene codes for a repressor protein. The function of lacX is not known. Promoter sites and directions are shown by the arrows, and potential transcriptional terminators are shown as hairpin loops. The number of amino acid residues for each protein is given. Adapted from de Vos et al., 1990.

141 171 310 326 105 586 568 468

galactose-6-P tagatose-6-P tagatose-1,6- EIIAlac EIIBClac phospho-b -

isomerase kinase diP aldolase galactosidase

Figure 2-14. The lac operon in lactococci. The operon consists of four structural genes (lacABCD) coding for enzymes of the tagatose pathway, two genes, lacFE, coding for lactose-specific PTS proteins, and the lacG gene coding for phospho-p-galactosidase. A divergently transcribed lacR gene codes for a repressor protein. The function of lacX is not known. Promoter sites and directions are shown by the arrows, and potential transcriptional terminators are shown as hairpin loops. The number of amino acid residues for each protein is given. Adapted from de Vos et al., 1990.

by ion gradients, and ATP-binding cassette (ABC) systems, fueled by ATP (Figure 2-15). Moreover, it is often the case that an organism uses a PTS for one sugar and a symport or ABC system for another.According to the Transport Classification system (www.tcdb.org), symport system transporters belong to a class of secondary carriers within the Major Facilitator Superfamily.

These symport systems consist of a membrane permease that has binding sites for both the substrate and a coupling ion, usually pro-tons.Transport of the substrate is driven by the ion gradient across the membrane. The most common ion gradient in bacteria is the proton gradient (called the proton motive force or

PMF), although there are also symport systems driven by sodium ion gradients (such as the melibiose system that exists in Lactobacillus casei).

The PMF is comprised of the sum of two components: (1) the charge difference (A^) across the membrane, where the outside is positive and the inside is negative; and (2) the chemical difference (ApH) across the membrane, where the proton concentration is high on the outside and low inside. The positively charged protons then flow "down" this gradient (i.e., toward the inside) in symport with the "uphill" intracellular accumulation of the solute. In lactic acid bacteria, symport systems exist for several sugars (based on biochemical and genetic evi-

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