Differential transcript accumulation and subcellular localization of Arabidopsis ribosomal proteins

Arabidopsis cytoplasmic ribosomes are an assembly of four rRNAs and 81 ribosomal proteins (RPs). With only a single molecule of each RP incorporated into any given ribosome, an adequate level of each RP in the nucleolus is a prerequisite for efficient ribosome biogenesis. Using Genevestigator (microarray data analysis tool), we have studied transcript levels of 192 of the 254 Arabidopsis RP genes, as well as the sub-cellular localization of each of five two-member RP families, to identify the extent to which these two processes contribute to the nucleolar pool of RPs available for ribosome biogenesis. While transcript levels from different RP genes show up to a 300-fold difference across the RP population, this difference is drastically reduced to ∼8-fold when considering RP gene families. Under various stimuli, while the transcript level for most RP genes remains unchanged some show a significantly increased or decreased level. Subcellular localization studies in tobacco not only showed differential targeting of RPs to the cytoplasm, nucleus and nucleolus, but also differential nucleolar import rates. This degree of variation in gene regulation and subcellular localization of RPs hints at the possibility of extra-ribosomal functions for some RP isoforms.