Abstract
The nucleotide sequence of the citC coding for the citrate carrier in several Salmonella serovars has been determined, and the amino acid sequence of the carrier protein was deduced. The predicted citrate carrier from Salmonella pullorum and Salmonella enteritidis consists of 446 amino acids with a molecular weight of 47,621, whereas that from Salmonella dublin is the same 446 amino acids with the slightly different molecular weight of 47,591, because 1 amino acid residue was substituted. The predicted proteins are highly hydrophobic (69% nonpolar amino acids). The hydropathy profile suggests that the proteins are composed of 11-12 hydrophobic membrane-spanning segments with two hydrophilic cores in the middle of the protein sequence. No homology in the nucleotide and amino acid sequences was found in the molecular structures of citA, citP, and tctI genes. The citC-coding citrate transport activity is Na(+)-dependent and specific for citrate only. The transcript from the citC gene was not detected in the total RNA from several Salmonella serovars except S. dublin in Northern blot analysis, although the promoter of the citC genes appeared to be functional in Escherichia coli and Salmonella typhimurium strains using the lacZ fusion assay. These results suggested that the citC gene-coding citrate carrier is probably a TctIII system such as that identified previously in S. typhimurium.
Highlights
The nucleotide sequence of the Citrate Carrier Gene (citC) coding for the to be controlled by a plasmid (3-5)
Detectedinthetotal RNAfrom several Salmonella serovars except S. dublin inNorthernblot analysis, the promoter of tchiteC genes appeared to be functional in Escherichia coli and Salmonella typhimurium strains using thelac[2] fusion assay
To isolate and characterize the DNA regions hybridized with the plasmid-borne citA gene, we cloned the DNA regionfrom the S . typhimurium chromosome to a vector plasmid using Cit+ability as aselection marker in an E. coli K-12 derivative
Summary
Strains-Chromosomal DNAs from five Salmonella serovars, S. typhimurium, S. enteritidis, S. abortusequi, S. dublin, and Bacterial Strains andPlasmids-Five Salmonellaserovars collected S. pullorum were digested with enzyme PstI. From the deletion analysis of pOH61 derivatives, the citC gene appeared to be located in the1,730bp internal HindIII-BamHI DNA region, The growth properties of E. coli S G l l carrying pOH61 were examined in Tanaka minimal medium containing sodium citrate or potassearch Laboratories and used accordingto themanufacturer's instruc- sium citrate as thesole carbon source. Coli SGll/pOH61 was dependent on the concentration of sodium ions (Fig. 5).Itthus appeared that citrate transport activity encoded by the citC gene was dependent on sodium ions. In these experiments, the totalconcentrations of NaCl and KC1 were kept constant at 100 mM so that there would be no osmotic difference
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