ISSN:
0091-7419
Keywords:
regulation
;
amino acid transport
;
mutants
;
leucine sensitivity
;
leucine
;
isoleucine
;
valine
;
Life Sciences
;
Molecular Cell Biology
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Biology
,
Chemistry and Pharmacology
,
Medicine
Notes:
Leucine is transported into E. coli cells by high-affinity transport systems (LIV-I and leucine-specific systems) which are sensitive to osmotic shock and require periplasmic binding proteins. In addition leucine is transported by a low-affinity system (LIV-II) which is membrane bound and retained in membrane vesicle preparations. The LIV-I system serves for threonine and alanine in addition to the 3 branched-chain amino acids. The LIV-II system is more specific for leucine, isoleucine, and valine while the high-affinity leucine-specific system has the greatest specificity.A regulatory locus, livR at minute 22 on the E. coli chromosome produces negatively regulated leucine transport and synthesis of the binding proteins. Valine-resistant strains have been selected to screen for transport mutants. High-affinity leucine transport mutants that have been identified include a LIV-binding protein mutant, livJ, a leucine-specific binding protein mutant livK and a nonbinding protein component of the LIV-I system, livH. A fourth mutant, livP, appears to be required only for the low-affinity LIV-II system. The existence of this latter mutant indicates that LIV-I and LIV-II are parallel transport systems. The 4 mutations concerned with high-affinity leucine transport form a closely linked cluster of genes on the E. coli chromosome at minute 74.The results of recent studies on the regulation of the high-affinity transport systems suggests that an attenuator site may be operative in its regulation. This complex regulation appears to require a modified leucyl-tRNA along with the transcription termination factor rho. Regulation of leucine transport is also defective in relaxed strains.Among the branched-chain amino acids only leucine produces regulatory changes in LIV-I activity suggesting a special role of this amino acid in the physiology of E. coli. It was shown that the rapid exchange of external leucine for intracellular isoleucine via the LIV-I system could create an isolucine pseudoauxotrophy and account for the leucine sensitivity of E. coli.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jss.400060315
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