THE GALACTOSE REGULON OF ESCHERICHIA-COLI

Galactose transport and metabolism in Escherichia coli involves a mult icomponent amphibolic pathway Galactose transport is accomplished by t wo different galactose-specific transport systems. At least four of th e genes and operons involved in galactose transport and metabolism hav e promoters containing similar regulatory sequences. These sequences a re recognized by at least three regulators, Gal repressor (GalR), Gal isorepressor (GalS) and cAMP receptor protein (CRP), which modulate tr anscription from these promoters. The negative regulators, GalR and Ga lS, discriminate between utilization of the high-affinity (regulated b y GalS) and low-affinity (regulated by GalR) transport systems, and mo dulate the expression of genes for galactose metabolism in an overlapp ing fashion. GalS is itself autogenously regulated and CRP dependent, while the gene for GalR is constitutive. The gal operon encoding the e nzymes for galactose metabolism has two promoters regulated by CRP in opposite ways; one (P1) is stimulated and the other (P2) inhibited by CRP. Both promoters are strongly repressed by GalR but weakly by GalS. All but one of the constituent promoters of the gal regulon have two operators. The gal regulon has the potential to coordinate galactose m etabolism and transport in a highly efficient manner, under a wide var iety of conditions of galactose availability.