Paralog-Specific Functions of RPL7A and RPL7B Mediated by Ribosomal Protein or snoRNA Dosage in Saccharomyces cerevisiae.

Most ribosomal proteins in Saccharomyces cerevisiae are encoded by two paralogs that additively produce the optimal protein level for cell growth. Nonetheless, deleting one paralog of most ribosomal protein gene pairs results in a variety of phenotypes not observed when the other paralog is deleted. To determine whether paralog-specific phenotypes associated with deleting [RPL7A][1] or [RPL7B][2] stem from distinct functions or different levels of the encoded isoforms, the coding region and introns of one paralog, including an intron-embedded snoRNA (small nucleolar RNA) gene, were exchanged with that of the other paralog. Among mutants harboring a single native or chimeric [RPL7][3] allele, expression from the [RPL7A][1] locus exceeded that from the [RPL7B][2] locus, and more [Rpl7a][1] was expressed from either locus than [Rpl7b][2]. Phenotypic differences in tunicamycin sensitivity, [ASH1][4] mRNA localization, and mobility of the Ty1 retrotransposon were strongly correlated with [Rpl7][3] and ribosome levels, but not with the [Rpl7][3] or snoRNA isoform expressed. Although Ty1 RNA is cotranslationally localized, depletion of [Rpl7][3] minimally affected synthesis of Ty1 Gag protein, but strongly influenced Ty1 RNA localization. Unlike the other processes studied, Ty1 cDNA accumulation was influenced by both the level and isoform of [Rpl7][3] or snoRNA expressed. These cellular processes had different minimal threshold values for [Rpl7][3] and ribosome levels, but all were functional when isoforms of either paralog were expressed from the [RPL7A][1] locus or both [RPL7][3] loci. This study illustrates the broad range of phenotypes that can result from depleting ribosomes to different levels. [1]: http://www.yeastgenome.org/locus/S000003044/overview [2]: http://www.yeastgenome.org/locus/S000006119/overview [3]: http://www.yeastgenome.org/locus/S000004947/overview [4]: http://www.yeastgenome.org/locus/S000001668/overview