Initiation of haemoglobin polypeptide chains

Abstract

1. 1. Ribosome fractions, prepared by differential centrifugation of a lysate of rabbit reticulocytes, contained different proportions of polyribosomes, ribosomal monomers (monosomes) and native subunits. In the cell-free system polyribosomes of all fractions synthesised on average one half polypeptide chain per constituent monomer. 2. 2. Chain synthesis de novo also occurred in the cell-free system, to a greater extent in the lighter of the fractions prepared by differential centrifugation. Since large quantities of monosomes were present in all fractions it is unlikely that monosomes are responsible for the initiation of polypeptide chains. An alternative hypothesis, that chain initiation requires the participation of native ribosomal subunits, is supported by several lines of evidence. 3. 3. The heaviest ribosome fraction, containing about 50 % polyribosomes but only 5 % subunits, had little capacity to initiate polypeptide chains. Chain initiation was increased by adding small amounts of a fraction rich in subunits. Zone sedimentation showed the stimulatory activity to sediment at about 50 S. 4. 4. Amongst a number of preparations of the ribosome fractions, a linear relationship could be demonstrated between chain initiation and subunit content, over a limited range. 5. 5. During incubation in the cell-free system the amount of ribosomal subunits declined. A parallel fall occurred in the capacity of the ribosome population to initiate polypeptide chains.