Regulation of amino acid transport in chick embryo heart cells II. Adaptive control sites for the “A mediation”

Abstract

The relative roles of transcription and translation of specific mRNA in the adaptive control of amino acid transport by the A mediation in chick embryo heart cells has been investigated, together with the mode of action of interfering amino acids. Results and conclusions based on the adopted experimental approach include the following: 1. Cells previously incubated in amino acid-free medium containing cycloheximide (phase of inhibited translation) subsequently exhibited a net increase of transport activity when transferred into a plain medium, a medium containing actinomycin D (phase of inhibited transcription), or a medium containing repressive concentrations of alanine (as representative amino acid of the A transport system). 2. The presence of alanine during prior incubation in cycloheximide prevented the increase of activity of the A transport system during subsequent incubation in cycloheximide-free actinomycin D-containing medium. Phenylalanine (as representative amino acid of the L mediation) was ineffective. 3. The synthesis of transport proteins of the A system (as estimated by measurements of transport activity) increased with the duration of cell preincubation in cycloheximide approaching equilibrium after about 40 min. The capacity to synthesize proteins rate-limiting to transport (as a measure of specific mRNA present) decreased during incubation of the cells in cycloheximide when actinomycin D or alanine were added. In both cases capacity decreased as a single exponential with comparable half-lives (approx. 170 min). 4. The degradation of transport proteins of the A mediation followed a single exponential with a half-life of about 200 min when cells were incubated with cycloheximide or puromycin. Actinomycin D did not alter the rate of degradation. Accelerated degradation (half-life of about 80 min) was observed when alanine was added to cells incubated in the presence of cycloheximide. 5. Actinomycin D did not promote the synthesis of transport proteins under conditions of derepressed (absence of added amino acids) or repressed (presence of alanine) transport activity of the A mediation.