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

Maternal inheritance determine the charge on the large subunit of fraction I protein whereas both parents of an interspecific F(1) 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 two kinds of protein molecules, one 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 two components of different charge by agarose gel electrophoresis but the protein from the F(1) hybrid plant between the two species was a single component. However, two-dimensional agarose gel electrophoresis showed that fraction I protein from an F(1) 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 two types could result in a mixture in which 99.2% of the population are hybrid molecules composed of seven types together with 0.39% containing small subunits of the maternal type and 0.39% containing those of the paternal type. In an F(1) hybrid plant in which three small subunit polypeptides randomize during assembly, a mixture of 45 types of fraction I protein macromolecules could be present.