Polypeptide composition of fraction I protein as an aid in the study of plant evolution : (chloroplast DNA, age of genera and species, origin of genomes, composition of proteins as affected by amphiploidy)
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Fraction 1 protein (F-1-P), found in all green plants, consists of eight large subunits (LS) coded by chloroplast DNA and eight small subunits (SS) coded by nuclear DNA. By electrofocusing in 8M urea many different species of F-1-P from all parts of the Plant Kingdom, the LS is invariably found to resolve into three polypeptides of different isoelectric points and the SS into from one to four polypeptides. During evolution of new plant species by amphiploidy the composition of the LS is always determined by the maternal parent, but both parents make equal contributions to the composition of the SS. In the case of N. tomentosiformis (n=12; 1SS) [male sign] x N. sylvestris (n=12; 1SS) [female sign] [rightwards arrow] N. tabacum (n=24; 2SS) [male sign] x N. glutinosa (n=12; 2SS) [female sign] [rightwards arrow] N. digluta (n=36; 4SS) a F-1-P evolved containing 4SS polypeptides. None of these arose by point mutation during speciation, although the SS of F-1-P of N, digluta could have eight differences in amino acid sequence compared to N. sylvestris or N. tomentosiformis, the differences being the consequence of amphiploidy. Using this example, together with F-1-P composition in parasexual hybrids, it is hypothesized that the genetic information for more than one SS polypeptide is non-allelic and most likely located on heterologous chromosomes. The study of F-1-P in 62 species of Nicotiana provides a model system whereby the LS is an indicator of the evolutionary age of one genus relative to another and the SS an indicator of the age of one species relative to another. The SS can also serve to designate a new species of plant, while the LS has been used to trace the origin of genomes in amphiploids.