Protein liganding to the activator cation of ribulosebisphosphate carboxylase.

Spinach leaf ribulosebisphosphate carboxylase forms a quaternary complex with CO2, carboxyarabinitol bisphosphate, and Cr2+ or Co2+. Oxidation of the cation in these complexes produces a protein--cation adduct which is sufficiently stable to be chromatographically isolated after enzyme denaturation. While stoichiometric levels of slowly exchanging cation can be specifically trapped after addition of protein denaturants as well as a vast molar excess of Mg2+, neither CO2 nor carboxyarabinitol bisphosphate remains bound to denatured protein under the conditions employed in these experiments. These observations demonstrate direct innersphere liganding of protein to the exchange-inert cation, which appears to bind at the site normally occupied by the physiologically active cation. Dimeric ribulosebisphosphate carboxylase from Rhodospirillum rubrum also forms a quaternary complex containing stoichiometric amounts of enzyme protomer, CO2, Co2+, and carboxyarabinitol bisphosphate. Lack of a small subunit in the R. rubrum enzyme does not impair binding of the components of the quaternary complex in a nonexchangeable mode. Substantial amounts of protein--cation adduct are recovered upon oxidation and denaturation of the R. rubrum complex, supporting the prediction that the large subunits of the octameric plant enzyme should be the sites of cation binding. The first direct proof for such a hypothesis has been generated by separation of protein subunits derived from a spinach quaternary complex and by the demonstration that the bound cation is associated with the large subunit.

Experimental approaches to embryonic erythropoiesis.

Erythroid systems in several developing organisms (including the human foetus and neonate, the chick embryo, the mouse embryo and the metamorphosing tadpole) have changing erythroid cell populations, changing haemoglobins and changing sites of erythropoiesis. The first two characteristics can be studied in vitro through organ cultures and cell cultures derived from early chick embryos. The prevention of early globin synthesis by specific inhibitors helps to define the mechanisms. There are interesting changes in the histone patterns during embryonic erythroid development and maturation. Changes in the composition and modification or erythroid chromatin proteins in the developing chick embryo and in the stimulated Friend erythroleukaemic system are discussed.