Species-specific proteins of the 50S subunit of the chloroplast ribosome in the genus Nicotiana.

The large subunits (50S) of chloroplast ribosomes were isolated from Nicotiana tabacum, a species of the Western Hemisphere, and from N. excelsior and N. gossei, Australian species. Their proteins were compared by two-dimensional gel electrophoresis. A pair of proteins (T12 and T12) observed in N. tabacum has electrophoretic mobilities which differ from those of a similarly migrating, and probably homologous, pair of proteins observed in N. excelsior and N. gossei. The species-specific proteins in N. tabacum differ slightly in electrophoretic mobilities based on both charge and molecular weight from those in N. excelsior and N. gossei. Tryptic digests of radioiodinated proteins reveal that the peptide maps of all six proteins are similar. These results suggest that chemically altered forms of one or more proteins of the 50S chloroplast ribosome subunit may exist in vivo.