A novel membrane-associated threonine permease encoded by the tdcC gene of Escherichia coli

Abstract

A novel L-threonine transport system is induced in Escherichia coi cells when incubated in amino acid-rich medium under anaerobic conditions. Genetic and biochemical analyses with plasmids harboring mutatons in the anaerobically expressed tdcABC operon indicated that the tdcC gene product was responsible for L-threonine uptake. Competition experiments revealed that the L-threoni transport system is also involved in L-serine uptake and is parially shared for L-leucine transport; L-alanine, L-valine, and L-sline did not affect L-threonine uptake. Transport of L-threonine was inhibited by the respiratory chain II KCN and carbonyl cyanide m-chlorophenylhydrazone and was Na+ independent. These results identify for the first time an E. coli gene encoding a permease specific for L-threouine-L-serine transport that is distinct from the previously described threonine-serine transport systems. A two-dimensional topological model predited from the amino acid composition and hydropathy plot showed that the TdcC polypeptide appears to be an integral membrane protein with several membrane-spanning domains exhibiting a striking similarity with other bacterial permeases.When incubated anaerobically in tryptone yeast extract (TYE) medium, Escherichia coli synthesizes biodegradative threonine dehydratase (EC 4.2.1.16) that catalyzes the dehydration of L-threonine and L-serine to ammonia and to the corresponding a-keto acids (32). Earlier, Hobert and Datta (18) described a synthetic medium (H4) which consists of four amino acids, threonine, serine, valine, and isoleucine, plus fumarate and cyclic AMP and supports high level of enzyme production during anaerobiosis. Recently, the structural gene for the dehydratase has been cloned on a 6.3-kilobase EcoRI DNA fragment (10, 13). A variety of experiments including DNA sequence analysis, deletion studies, minicell expression, and insertion mutagenesis (13,26,27) indicated that tdcB encoding threonine dehydratase is part of the tdcABC operon that harbors two additional genes, tdcA and tdcC (Fig. 1). In addition, genetic analysis of the cloned DNA revealed that efficient expression of the operon requires the product of a regulatory gene, tdcR, situated upstream of the tdc promoter in opposite transcriptional orientation (28).The enzymatic function of the tdcB gene product in threonine degradation is well established (10); however, the physiological significance of TdcA and TdcC is not yet understood. Published reports (cited in reference 22) indicate that most catabolic pathways, especially those involved in the utilization of various amino acids and sugars, include a specific protein(s) for transport of metabolites into the cell and that some catabolic operons, such as lac and tna, harbor genes coding for their respective permeases lacY and tnaB (4, 30). For the tdc operon, both threonine and serine are required for expression of the tdc genes in the synthetic H4 medium (18), and thus far no E. coli gene specific for L-threonine and L-serine transport has been identified. These consider...

l -Threonine Export: Use of Peptides To Identify a New Translocator from Corynebacterium glutamicum

“…In a special environment, such as the bird intestine, the export of selected low-molecular-weight compounds might be advantageous. This scenario, together with the fact that ThrE also accepts L-serine as a substrate, a feature typical of L-threonine uptake carriers like TdcC and SstT of E. coli (30,46), leads us to assume that ThrE of C. glutamicum is structurally designed for the export of small solutes that have a structure similar to that of L-threonine.…”

“…It is well-known that the Lthreonine uptake systems of E. coli, encoded by tdcC and sstT, also catalyze L-serine uptake (30,46). We were therefore interested in analyzing whether this is also true of the new carrier which translocates L-threonine from the interior of the cell to the environment of the cell.…”

Functional analysis of the tdcABC promoter of Escherichia coli: roles of TdcA and TdcR

“…The operon contains two structural genes, tdcB and tdcC, encoding, respectively, the biodegradative threonine dehydratase (EC 4.2.1.16) catalyzing the dehydration of L-threonine and L-serine to ammonia and the corresponding ox-keto acids and a membrane-associated Lthreonine-L-serine permease (2,6,23). The regulatory gene tdcA, proximal to the tdc promoter, encodes a protein homologous to the LysR family of transcriptional activators and is required for maximal induction of the tdc operon (3,5).…”

Involvement of Fnr and ArcA in anaerobic expression of the tdc operon of Escherichia coli

“…1A), is implicated in transport and metabolism of the amino acids during anaerobic growth to provide a source of metabolic energy (3,6,7,37). Upstream of tdcABC and transcribed in opposite orientation is tdcR, which specifies a small protein that is essential for tdc transcription (10,33).…”