The scr regulon of pUR400 and the chromosomally encoded scr regulon of Klebsiella pneumoniae KAY2026 are both negatively controlled by a specific repressor (ScrR). As deduced from the nucleotide sequences, both scrR genes encode polypeptides of 334 residues (85.5% identical base pairs, 91.3% identical amino acids), containing an N-terminal helix-turn-helix motif. Comparison with other regulatory proteins revealed 30.6% identical amino acids to FruR, 27.0% to Lacl and 28.1% to GalR. Six scrRs super-repressor mutations define the inducer-binding domain. The scr operator sequences were identified by in vivo titration tests of the sucrose repressor and by in vitro electrophoretic mobility shift assays. D-fructose, an intracellular product of sucrose transport and hydrolysis, and D-fructose 1-phosphate were shown to be molecular inducers of both scr regulons. An active ScrR-FruR hybrid repressor protein was constructed with the N-terminal part of the sucrose repressor of K. pneumoniae and the C-terminal part of the fructose repressor of Salmonella typhimurium LT2. Gel retardation assays showed that the hybrid protein bound to scr-specific operators, and that D-fructose 1-phosphate, the inducer for FruR, was the only inducer. In vivo, neither the operators of the fru operon nor of the pps operon, the natural targets for FruR, were recognized, but the scr operators were. These data and the data obtained from the super-repressor alleles confirm previous models on the binding of repressors of the Lacl family to their operators.

A typical eubacterium carries a battery of substrate transport systems which are the ultimate pacemakers for growth. These systems reflect a billion year old selection for coping with rapidly changing conditions in the environment and each of them is optimised for specific growth conditions. Metabolic pathways in combination with transport systems can be interpreted as transient sensory systems, where a transport system corresponds to a sensor for external stimuli. Characteristics is a tightly linked common control between a carbohydrate metabolic pathway and the corresponding transport system. Many of the observed growth phenomena are a direct result of adaptation and regulation of transport capacity to rapid changes in environmental conditions. Some of the better understood examples are discusses. Nevertheless, knowledge on bacterial carbohydrate transport under environmental conditions as documented in the literature is still scarce.

Of four putative intramembrane charge pairs in lactose permease, only three are conserved in the homologous sucrose permease of Escherichia coli [Bockmann, J., Heuel, H., & Lengeler, J. W. (1992) Mol. Gen. Genet. 235, 22-32]. The missing charge pair was introduced into wild-type sucrose permease by site-directed mutagenesis of Asn234 (helix VII) and Ser356 (helix XI). Individual replacement of either residue with a charged amino acid abolishes active sucrose transport with the exception of the Asn234-->Asp mutant. However, simultaneous replacement of Asn234 with Asp or Glu and Ser356 with Arg or Lys results in high activity. Thus, an acidic residue at position 234 rescues the activity of the Ser356-->Arg or Ser356-->Lys mutant, and a basic residue at position 356 rescues the activity of the Asn234-->Glu mutant. Furthermore, when expressed at a relatively low rate, the double mutant Asn234-->Asp/Ser356-->Arg is present in the membrane in a significantly greater amount than wild-type, suggesting that the charge pair improves insertion of sucrose permease into the membrane. The results indicate that helices VII and XI of sucrose permease are in close proximity and that a charge pair interaction can be established between residues 234 (helix VII) and 356 (helix XI). However, interchange of the acidic residue at position 234 with the basic residue at position 356 abolishes sucrose transport.(ABSTRACT TRUNCATED AT 250 WORDS)