Evolutionary divergence of ribosomal protein paralogs in Arabidopsis

Abstract

The majority of proteins in plants are synthesized by the cytoplasmic ribosome. In Arabidopsis, this massive (~3.2 MDa), two-subunit (40S small subunit [SSU] and 60S large subunit [LSU]) enzyme is comprised of 81 ribosomal proteins (RPs; 33 SSU, 48 LSU) that assemble around a catalytic core of 4 ribosomal RNAs (rRNA; 1 SSU, 3 LSU).1,2 None of the Arabidopsis RPs are encoded by single genes, but rather derive from families of 2–7 members that encode nearly-identical isoforms, are independently regulated, dispersed throughout the genome, and largely all transcriptionally active.2 To gain an understanding of why so many plant RP paralogs exist, we recently analyzed function and localization of the two-member Arabidopsis RPL23a family (RPL23aA and RPL23aB).3 RPL23aB was found to be unnecessary for normal development, while a relatively small decrease in transcript levels of RPL23aA resulted in development of a severe phenotype. Isoforms exhibited differences in nucleolar-targeting, which may result from disparity in putative nuclear/nucleolar localization signals (NLS/NoLSs). We postulate a role for ribosome biogenesis in the primary regulation of auxin homeostasis and plant development, and discuss properties of high efficiency NoLSs.Addendum to: Degenhardt RF, Bonham-Smith PC. Arabidopsis ribosomal proteins RPL23aA and -B are differentially targeted to the nucleolus and are disparately required for normal development. Plant Physiol 2008; In press.