Regulation of hemoglobin synthesis. Effect of concentration of messenger ribonucleic acid, ribosome subunits, initiation factors, and salts on ratio of alpha and beta chains synthesized in vitro.

Protein fractions washed from the free small ribosome subunits present in a reticulocyte lysate, with 40 S and 60 S ribosome subunits and 9 S RNA from reticulocyte polysomes, promote the initiation of synthesis in uifro of both the LY and j3 chains of rabbit hemoglobin. Changing relative concentrations of 9 S RNA, polysome-derived ribosome subunits, and difierent initiation factor fractions cause changes in the ratio of (Yto @globin product synthesized. Increasing 9 S RNA concentration with constant ribosome subunit and initiation factor concentration causes a decreasing ratio of a to fl product, whereas increasing ribosome subunit concentration with constant 9 S RNA and initiation factor concentration promotes an increasing ratio of a! to /3 product. Increasing levels of derived 40 S subunits relative to derived 60 S subunits with constant 9 S RNA and initiation factor concentration causes an increase in ratio of a! to j3 product. Increasing levels of derived 60 S subunits relative to 40 S subunits causes a small decrease in ratio of (Yto @-globin synthesized. In addition, the ratio of (Yto @globin synthesized is afIected by the relative amounts of two separate fractions of the salt wash from the free small subunits. Increasing levels of a fraction containing a factor which inhibits the spontaneous association of polysome-derived 40 S subunits with 69 S subunits into single ribosomes causes an increase in the ratio of (Yto j3-globin product synthesized. The Mg2+ and K+ concentrations have marked effects on the ratio of (Yto j3-globin synthesized. The results are discussed in terms of (a) different affinities of the initiation sites of LYand j3-mRNA for ribosome subunits active in initiation; (b) tierent availabilities of initiation sites of OIand /3-mRNA to the initiating ribosome subunits; and (c) the concentration of ribosome subunits available to interact with the initiation sites of mRNA.