The photoautotrophic cyanobacterium Anacystis nidulans was used to investigate the membrane transport of branched-chain,
neutral amino acids and its dependence on photosynthetic reactions. The uptake of alpha-amino [1-14C]
isobutyric acid and L-[1-14C]
leucine followed Michaelis, Menten kinetics and resulted in an energy-dependent accumulation. As in bacteria, different uptake systems for
neutral amino acids were present: two DAG (D-
alanine, aminoisobutyric
acid, and
glycine) systems responsible for uptake of alpha-amino [1-14C]
isobutyric acid, and one LIV (
leucine,
isoleucine, and
valine) system, responsible for uptake of
leucine. The low-affinity DAG system seemed to be dependent on the presence of Na+
ions. Uptake was enhanced by white light and by monochromatic light of 630 nm. In far red light (717 nm) with and without
nitrogen flushing, considerable uptake dependent on light intensity and inhibition by
dibromothymoquinone and by high concentrations of KCN were observed. Therefore, the energy generated by
photosystem I reactions only could perform this membrane transport. The
proton translocator carbonylcyanide m-chlorophenylhydrazone and N,N-
dicyclohexylcarbodiimide as an
ATPase inhibitor reduced
amino acid uptake to a high degree. A pH dependence of aminoisobutyric
acid and
leucine uptake was obvious, with a maximum at pH 6 to 7 and some at a pH as high as 9.5. At higher pH, increasing concentrations of Na+ K+ and also of
triphenylmethylphosphonium ions inhibited the transport of aminoisobutyric
acid. These findings are consistent with the assumption that
ATP from photosynthetic reactions drives a membrane-bound
proton-translocating ATPase producing a proton motive force, consisting at higher pH chiefly in a delta psi amount, which promotes a secondary active H+ or Na+/
amino acid symport carrier.