The metabolism of gluconate in Escherichia coli. The subsidiary system and the nature of the gntS gene

Abstract

The transport and phosphorylation of gluconate in E. coli occurs through two systems (GntI and GntII) which duplicate activities. bioH-asd deletion mutants do not grow on media with gluconate as sole carbon source because they lack the GntI system and do not express GntII. Although E. coli c177 is a delta (bioH-asd) mutant, it carries the pyrB linked mutation gnt177 that enables it to metabolize this substrate through inducible expression of the GntII system. Several gntS derivatives which are unable to grow on gluconate were isolated from E. coli C177 by spontaneous curing of the transposon Tn10 previously inserted at the gntS locus (zjf::Tn10, min 95.3). A representative gntS mutant, E. coli TI141A retained the ability to take up gluconate but had lost the thermosensitive gluconokinase activity (gene gntV, min 96.9). Furthermore, it could be demonstrated that gntV is repressed in E. coli TI141A. The results indicate that gntS might specify a trans-acting positive regulator involved in the control of at least the expression of the thermosensitive gluconokinase (GntII), instead of a gluconate uptake system as it was previously postulated. Likewise, these results can be used to reconsider whether the locus altered by the gnt177 lesion is allelic with that of the GntII permease instead of a regulator, as it was originally postulated.

Sequence Analysis of the GntII (Subsidiary) System for Gluconate Metabolism Reveals a Novel Pathway for l -Idonic Acid Catabolism in Escherichia coli

“…Genetic evidence for the presence of two systems for gluconate transport and phosphorylation was first provided by Bächi and Kornberg (2). Gluconate-fermenting pseudorevertants of E. coli HfrG6⌬MD2, a GntI deletion mutant (bioH-asd) which cannot grow on gluconate, were selected after extended incubation on minimal medium containing gluconate (11)(12)(13)22). The pseudorevertant strains were found to have induced a thermosensitive gluconate kinase and an alternative gluconate transporter when growing on gluconate.…”

“…A mutation affecting the subsidiary gluconate transporter was designated gntS (2), but it was never proven whether the gntS locus is a structural or regulatory gene, although it has been suggested that the gntS product positively controls expression of idnK (gntV) (2,11,13). Recent evidence apparently confirms the location of gntS upstream of fbp in the 95-min region and further supports the conclusion that gntS is a regulatory locus (12). However, examination of the genomic sequence in the 95-min region does not provide any further insights into the nature of gntS.…”

“…However, examination of the genomic sequence in the 95-min region does not provide any further insights into the nature of gntS. It is now believed that the subsidiary gluconate transporter is encoded by idnT (gntW), which is adjacent to the proven location of idnK at 96.8 min (12,23). The nature and role of gntS remain obscure.…”

“…It was never established during the previous studies how the GntII system is regulated. The GntII system was thought to have evolved as a subsidiary pathway for gluconate catabolism (12). If so, the GntII system would need contain only a gluconate transporter and gluconate kinase.…”

Characterization and Use of Catabolite-Repressed Promoters from Gluconate Genes in Corynebacterium glutamicum

“…Both systems are transcriptionally regulated, i.e., induced by gluconate and repressed by glucose (9,58,60). In E. coli, another subsidiary gluconate system (GntII) has been previously described (30), but it is now considered the catabolic pathway for L-idonate where D-gluconate is an intermediary (4); the idn idonate operon (the former GntII) is repressed by a regulatory protein (IdnR) and globally repressed by the carbon source (5).…”