The ribosomal protein S16 (RPS16), the product of the
rps16, is generally encoded in the chloroplast genomes of flowering plants. However, it has been reported that chloroplast-encoded RPS16 in mono- and dicotyledonous plants has been substituted by the product of nuclear-encoded
rps16, which was transferred from the mitochondria to the nucleus before the early divergence of angiosperms. Current databases show that the chloroplast-encoded
rps16 has become a pseudogene in four species of the Brassicaceae (
Aethionema grandiflorum,
Arabis hirsuta,
Draba nemorosa, and
Lobularia maritima). Further analysis of
Arabidopsis thaliana and its close relatives has shown that pseudogenization has also occurred via the loss of its splicing capacity (
Arabidopsis thaliana and
Olimarabidopsis pumila). In contrast, the spliced product of chloroplast-encoded
rps16 is observed in close relatives of
Arabidopsis thaliana (
Arabidopsis arenosa,
Arabidopsis lyrata, and
Crucihimalaya lasiocarpa). In this study, we identified the different functional status of
rps16 in several chloroplast genomes in the genus
Arabidopsis and its close relatives. Our results strongly suggest that nuclear- and chloroplast-encoded
rps16 genes coexisted for at least 126 million years. We raise the possibility of the widespread pseudogenization of
rps16 in the angiosperm chloroplast genomes via the loss of its splicing capacity, even when the
rps16 encoded in the chloroplast genome is transcriptionally active.
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