Evolution of the NH2- and COOH-terminal extensions of chloroplast ribosomal protein S18. Nucleotide sequence of pea and rye chloroplast rps 18 genes.

An unusual, variably repeated heptapeptide motif is present in most chloroplast ribosomal protein S18 sequences (Weglöhner and Subramanian, FEBS Lett. 269, 193-197, 1991), whereas it is absent in bacterial, cyanelle, and in the chloroplast S18 of the lower plant liverwort. In order to understand the evolution of this higher plant-specific motif, we have cloned and sequenced chloroplast rps18 genes from pea, a dicot plant of the large legume family and rye, a monocot plant with temperature-sensitive chloroplast ribosome formation. The derived amino acid sequence of pea S18 protein shows two and that of rye seven repeats of this motif. We also show that a different heptapeptide motif is discernible in the recently published chloroplast S18 sequence of Pinus thunbergii (a gymnosperm), which can however be derived convergently from a putative progenitor of angiosperm-gymnosperm chloroplast S18. The presence of a 3-fold repeat of an asparagine-rich heptapeptide in the C-terminal extensions of all cereal S18 is also shown here. The results are further discussed in terms of possible origin of these repeats and the ribosomal protein evolution in general.