ON THE USE OF ACTINOMYCIN FOR OBSERVING THE TURNOVER OF RIBONUCLEIC ACID

Phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase: Characterisation of the concomitant changes in enzyme activities in illuminated potato tuber discs

Phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and cinnamic acid 4-hydroxylase (CA4H, EC 1.14.13.11) undergo concomitant increases in activity after a 2 h lag following disc preparation and illumination. The increases in PAL and CA4H activity can be inhibited by actinomycin-D, cordycepin and cycloheximide. It is demonstrated that the concomitant increases in enzyme activity stem from the simultaneous stimulation of the respective actinomycin-D-sensitive steps followed by parallel transmission of these stimuli through the cordycepin- and cycloheximide-sensitive steps. The events between the actinomycin-D- and cordycepin-sensitive steps are rate limiting in the expression of the stimulations in terms of increased enzyme activities. Delayed transfer of discs to actinomycin-D or cordycepin 'superinduces' PAL but not CA4H activity. This superinduction is related to the inhibition by cycloheximide of the subsequent decline in PAL activity and is interpreted in terms of the transcription and translation of the mRNA coding for an inactivator-protein of PAL.

The regulatory process in the de-repression of enzyme synthesis. Alkaline phosphatase of Bacillus subtilis

1. The kinetics of de-repression of alkaline phosphatase in Bacillus subtilis were studied after the removal of P(i). Enzyme activity appeared about 10min. after removal of P(i), whereas ;enzyme-forming potential' appeared after 6min. 2. Protein synthesis is not impaired for at least 20min. on removal of P(i), but RNA synthesis is considerably diminished. 3. Adding chloramphenicol to cells without P(i), just at the time they start to make enzyme-forming potential, does not affect the differential rate of enzyme synthesis compared with total protein. Enzyme-forming potential accumulates to about normal levels in the presence of chloramphenicol, even though peptide-bond formation is inhibited by more than 95%. 4. Similar experiments performed with actinomycin C show more complex effects. Actinomycin initially prevents RNA synthesis and also the synthesis of enzyme-forming potential. After some minutes RNA synthesis resumes at a low rate, to be followed 4min. later by enzyme synthesis. Enzyme-forming potential can accumulate in the presence of actinomycin after the resumption of RNA synthesis. Protein synthesis, initially inhibited by actinomycin as a consequence of the effect on RNA synthesis, is later directly inhibited by actinomycin. 5. Adding actinomycin to de-repressed cells already making enzyme stops enzyme synthesis within 4-5min. Enzyme synthesis resumes, as before, 4min. after the resumption of RNA synthesis. 6. Adding P(i) together with actinomycin to de-repressed cells synthesizing enzyme does not result in a lower yield of enzyme compared with actinomycin alone. 7. Actinomycin is less effective an inhibitor of RNA and protein synthesis in P(i)-starved cells if P(i) is also added. 8. These results are discussed in view of the three main models for the regulation of enzyme induction: regulation at the level of transcription only, at translation only, or a coupled model in which transcription requires concomitant translation. It is concluded that the present evidence most powerfully supports the model of transcriptional regulation.

Protein synthesis by long-lived messenger ribonucleic acid in bacteria

1. Experiments were performed to investigate two hypotheses about the function of long-lived messenger RNA in bacteria. After RNA synthesis had been stopped by the addition of actinomycin, continuing protein synthesis was used as a measure of persistent messenger RNA. 2. The hypothesis that messenger RNA responsible for the synthesis of membrane protein is exceptionally long-lived was tested in experiments with protoplasts of Bacillus megaterium. However, this messenger RNA proved to be of approximately average stability. 3. The hypothesis that long-lived messenger RNA is responsible for the synthesis of constitutive proteins was tested by comparing the synthesis of penicillinase in an inducible and a constitutive strain of Bacillus licheniformis. After the addition of actinomycin, penicillinase synthesis continued for far longer in the constitutive than in the inducible strain. This difference is attributed to a difference in stability of the penicillinase-messenger RNA in the two strains, which does not extend to all messenger RNA indiscriminately. 4. A model is tentatively proposed to account for the altered stability of messenger RNA in the constitutive mutant.