Involvement of hemin, a stimulatory fraction from ribosomes and a protein synthesis inhibitor in the regulation of hemoglobin synthesis

For maximum globin synthesis in lysate cell-free systems prepared from rabbit reticulocytes, there is a requirement for both hemin and a fraction derived from the ribosome by salt treatment (I fraction). A previously described inhibitor of protein synthesis (Q fraction), which accumulates during incubation of the lysate system, is shown to be without effect on completion and release of nascent chains, suggesting that it is acting at the site of chain initiation. Inhibition of globin synthesis by Q fraction is not reversed by addition of salt-treated ribosomes but is blocked and reversed by the addition of the I fraction, suggesting that an I fraction component is limiting in systems containing Q fraction. Several modes of regulation of protein synthesis involving hemin, I fraction and Q fraction are discussed in the light of kinetic data obtained with these systems, and in the light of the finding that intact cells incubated in the absence of added iron accumulate an inhibitor with properties similar to Q fraction desscribed above.