Affiliation:
1. Research School of Biological Sciences, Australian National University, Canberra, Australia
Abstract
Phenylalanine, tyrosine, and tryptophan were taken up into cells of
Escherichia coli
K-12 by a general aromatic transport system. Apparent Michaelis constants for the three amino acids were 4.7 × 10
−7
, 5.7 × 10
−7
, and 4.0 × 10
−7
m
, respectively. High concentrations (> 0.1 m
m
) of histidine, leucine, methionine, alanine, cysteine, and aspartic acid also had an affinity for this system. Mutants lacking the general aromatic transport system were resistant to
p
-fluorophenylalanine, β-2-thienylalanine, and 5-methyltryptophan. They mapped at a locus,
aroP
, between
leu
and
pan
on the chromosome, being 30% cotransducible with
leu
and 43% cotransducible with
pan
. Phenylalanine, tyrosine, and tryptophan were also transported by three specific transport systems. The apparent Michaelis constants of these systems were 2.0 × 10
−6
, 2.2 × 10
−6
, and 3.0 × 10
−6
m
, respectively. An external energy source, such as glucose, was not required for activity of either general or specific aromatic transport systems. Azide and 2,4-dinitrophenol, however, inhibited all aromatic transport, indicating that energy production is necessary. Between 80 and 90% of the trichloroacetic acid-soluble pool formed from a particular exogenous aromatic amino acid was generated by the general aromatic transport system. This contribution was abolished when uptake was inhibited by competition by the other aromatic amino acids or by mutation in
aroP
. Incorporation of the former amino acid into protein was not affected by the reduction in its pool size, indicating that the general aromatic transport system is not essential for the supply of external aromatic amino acids to protein synthesis.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology