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pubmed:abstractText |
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.
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