Random assembly of different kinds of small subunit polypeptides during formation of fraction I protein macromolecules

Abstract

Maternal inheritance determines the charge on the large subunit of fraction I protein whereas both parents of an interspecific F1 hybrid plant make an equal contribution to the charge of the small subunit. Experiments were performed to test whether fraction I protein in the hybrid plant was a hybrid molecule or 2 kinds of protein molecules, 1 composed of large subunits combined with small subunits of the male type and the other composed of large subunits combined with small subunits of the female type. A mixture of fraction I proteins from Nicotiana langsdorffii and N. bonariensis was resolved into 2 components of different charge by agarose gel electrophoresis but the protein from the F1 hybrid plant between the 2 spp. was a single component. However, 2 dimensional agarose gel electrophoresis showed that fraction I protein from an F1 hybrid could be highly heterogeneous in charge compared to a fraction I protein containing a single small subunit polypeptide that was homogeneous in charge. Evidently, charge heterogeneity results from random self-assembly of the small subunit polypeptides. Random self-assembly of small subunits of 2 types could result in a mixture in which 99.2% of the population are hybrid molecules composed of 7 types together with 0.39% containing small subunits of the maternal type and 0.39% containing those of the paternal type. In an F1 hybrid plant in which 3 small subunit polypeptides randomize during assembly, a mixture of 45 types of fraction I protein macromolecules could be present.