Control of globin synthesis by hemin An intermediate form of the translational repressor in rabbit reticulocyte lysates

Abstract

The regulation of globin synthesis by hemin in rabbit reticulocytes and reticulocyte lysates appears to be mediated by a translational repressor, which is formed in the post-ribosomal supernatant fraction in the absence of hemin and inhibits polypeptide-chain initiation. This report demonstrates that the repressor is formed in three stages denoted reversible, intermediate, and irreversible. The reversible hemin-controlled repressor is inactivated by hemin, but the latter two forms are not. When added to reticulocyte cell-free systems incubated in the absence of hemin, the reversible and intermediate forms of the repressor only transiently inhibit protein synthesis, whereas the irreversible form exerts progressively more inhibition with time of incubation. The intermediate hemin-controlled repressor functions only transiently, apparently because it becomes inactivated in the course of inhibiting protein synthesis. The intermediate repressor appears to bind hemin, and this suppresses its inhibitory effect at 25°C but not at 34°C. Hemin prevents the conversion of the intermediate to the irreversible form of the repressor at temperatures above 25°C. The intermediate hemin-controlled repressor may play a physiological role in the regulation of protein synthesis in intact cells, since if its formation becomes blocked by hemin after a period of hemin deficiency, protein synthesis can still recover.