Hypothesis for the evolutionary origin of the chloroplast ribosomal protein L21 of spinach

A full size cDNA clone encoding the chloroplast ribosomal protein L21 from spinach is presented. The identity of the clone and the location of the transit peptide processing site were determined by comparison with the N-terminal amino acid sequence of the spinach chloroplast protein CS-L7 previously identified. L21 r-protein sequences from spinach, Marchantia polymorpha and Escherichia coli are compared. Quite surprisingly, the data do not suggest that the rpl21 nuclear gene from spinach was derived through intracellular gene transfer from the chloroplast genome. The possibility of a mitochondrial origin for rpl21 gene of spinach is discussed.