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Methionine transport in Salmonella typhimurium: evidence for at least one low-affinity transport system
Abstract:
The systems which transport methionine in Salmonella typhimurium LT2 have been studied. Fourteen mutants, isolated by three different selection procedures, had similar growth characteristics and defects in the specific transport process showing a Km of 0.3 microM for L-methionine, and therefore lack the high-affinity, metP transport system. The sites of mutation in four of the mutants were shown by P1-mediated transduction to be linked (0.3 to 1.1%) with a proline marker located at unit 7 on the S. typhimurium chromosome. The high-affinity system was subject to both repression and transinhibition by methionine, and it may also be regulated by the metJ and metK genes. There appeared to be at least two additional transport systems with relatively low affinities for methionine in the metP763 mutant strain, with apparent Km values for methionine of 24 microM and approximately 1.8 mM. The latter system, with a very low affinity for methionine, was inhibited by leucine. In addition, methionine inhibited leucine transport, suggesting that one of the low-affinity methionine transport systems may actually be a leucine transport system.
Insights
Researchers studied methionine transport in Salmonella typhimurium LT2, identifying a high-affinity system (metP) and uncovering at least two low-affinity systems. One low-affinity system appears to be a leucine transporter.
Area of Science:
- Microbiology
- Molecular Biology
- Biochemistry
Background:
- Methionine is an essential amino acid crucial for protein synthesis and cellular metabolism.
- Efficient transport systems are vital for microorganisms to acquire essential nutrients like methionine.
- Salmonella typhimurium LT2 serves as a model organism for studying bacterial transport mechanisms.
Purpose of the Study:
- To investigate the systems responsible for methionine transport in Salmonella typhimurium LT2.
- To characterize the properties and regulation of methionine transport pathways.
- To identify potential overlaps or interactions between methionine and other amino acid transport systems.
Main Methods:
- Isolation and characterization of Salmonella typhimurium LT2 mutants with defects in methionine transport.
- Determination of kinetic parameters (Km values) for L-methionine uptake.
- Genetic mapping of mutations using P1-mediated transduction.
- Analysis of transport system regulation by methionine and other amino acids.
Main Results:
- Fourteen mutants lacking the high-affinity metP transport system (Km = 0.3 μM) were identified.
- Mutations were mapped to a specific chromosomal locus linked to a proline marker.
- The high-affinity system is regulated by methionine (repression and transinhibition) and potentially by metJ and metK genes.
- Two low-affinity methionine transport systems (Km = 24 μM and ~1.8 mM) were observed in a metP mutant.
- The low-affinity system with a very low affinity was inhibited by leucine, and methionine inhibited leucine transport.
Conclusions:
- Salmonella typhimurium LT2 possesses a high-affinity methionine transport system (metP) and at least two low-affinity systems.
- The metP system is subject to complex regulatory mechanisms involving methionine and specific genes.
- One of the low-affinity methionine transport systems may be a shared or identical system for leucine transport, indicating nutrient transporter cross-regulation.