FIG. 4.

Diagram of the utilization of lactose, galactose, and N-acetylglucosamine (GlcNAc) as sole carbon sources and related pathways discussed in this work. The gene products indicated are LacY, lactose transporter; LacZ, β-galactosidase; GalP, low-affinity galactose transporter; MglABC, high-affinity galactose transporter; GalK, galactokinase; GalT, galactose 1-phosphate uridylyltransferase; GalE, UDP-galactose-4-epimerase; Pgm, phosphoglucomutase; Pgi, phosphoglucoisomerase; NagE, GlcNAc transporter; NagA, GlcNAc-6-phosphate deacetylase; NagB, GlcN-6-phosphate deaminase; ManXYZ, glucosamine transporter; GlmS, GlcN-6-phosphate synthase; GlmM, phosphoglucosamine mutase; GlmU, GlcN 1-phosphate acetylase; and UDP-GlcNAc synthase. As indicated by a bent arrow, LacY is known to transport galactose (15, 50), and elevated LacY expression apparently allows fast growth of MG1655 (CGSC 6300) on galactose as the carbon source (see text). Cells grown on lactose as the carbon source apparently synthesize inducers of the maltose regulon endogenously. Such synthesis from glucose and glucose 1-phosphate has been proposed previously (1, 9, 51), although the enzymes involved have not been defined. The product of gene ycjT does not appear to be solely responsible for endogenous synthesis of inducer (see text). NagC (not shown) regulates both the synthesis and degradation of GlcNAc (see Discussion). Inactivation of NagC allows fast growth of MG1655 on galactose, apparently by decreasing endogenous synthesis of GlcN 6-phosphate and thus increasing the flux of fructose 6-phosphate through central metabolism (see Discussion).