Introns provide a platform for intergenic regulatory feedback of RPL22 paralogs in yeast

Mechanisms and regulation of spliceosome-mediated pre-mRNA splicing in Saccharomyces cerevisiae

Pre-mRNA splicing, the removal of introns and ligation of flanking exons, is a crucial step in eukaryotic gene expression. The spliceosome, a macromolecular complex made up of five small nuclear RNAs (snRNAs) and dozens of proteins, assembles on introns via a complex pathway before catalyzing the two transesterification reactions necessary for splicing. All of these steps have the potential to be highly regulated to ensure correct mRNA isoform production for proper cellular function. While Saccharomyces cerevisiae (yeast) has a limited set of intron-containing genes, many of these genes are highly expressed, resulting in a large number of transcripts in a cell being spliced. As a result, splicing regulation is of critical importance for yeast. Just as in humans, yeast splicing can be influenced by protein components of the splicing machinery, structures and properties of the pre-mRNA itself, or by the action of trans-acting factors. It is likely that further analysis of the mechanisms and pathways of splicing regulation in yeast can reveal general principles applicable to other eukaryotes. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing.

Genetic compensation between ribosomal protein paralogs mediated by a cognate circular RNA

Inter-regulation between related genes, such as ribosomal protein (RP) paralogs, has been observed to be important for genetic compensation and paralog-specific functions. However, how paralogs communicate to modulate their expression levels is unknown. Here, we report a circular RNA involved in the inter-regulation between RP paralogs RpL22 and RpL22-like during Drosophila spermatogenesis. Both paralogs are mutually regulated by the circular stable intronic sequence RNA (sisRNA) circRpL22(NE,3S) produced from the RpL22 locus. RpL22 represses itself and RpL22-like. Interestingly, circRpL22 binds to RpL22 to repress RpL22-like, but not RpL22, suggesting that circRpL22 modulates RpL22's function. circRpL22 is in turn controlled by RpL22-like, which regulates RpL22 binding to circRpL22 to indirectly modulate RpL22. This circRpL22-centric inter-regulatory circuit enables the loss of RpL22-like to be genetically compensated by RpL22 upregulation to ensure robust male germline development. Thus, our study identifies sisRNA as a possible mechanism of genetic crosstalk between paralogous genes.

Truncating the spliceosomal 'rope protein' Prp45 results in Htz1 dependent phenotypes

Spliceosome assembly contributes an important but incompletely understood aspect of splicing regulation. Prp45 is a yeast splicing factor which runs as an extended fold through the spliceosome, and which may be important for bringing its components together. We performed a whole genome analysis of the genetic interaction network of the truncated allele of PRP45 (prp45(1-169)) using synthetic genetic array technology and found chromatin remodellers and modifiers as an enriched category. In agreement with related studies, H2A.Z-encoding HTZ1, and the components of SWR1, INO80, and SAGA complexes represented prominent interactors, with htz1 conferring the strongest growth defect. Because the truncation of Prp45 disproportionately affected low copy number transcripts of intron-containing genes, we prepared strains carrying intronless versions of SRB2, VPS75, or HRB1, the most affected cases with transcription-related function. Intron removal from SRB2, but not from the other genes, partly repaired some but not all the growth phenotypes identified in the genetic screen. The interaction of prp45(1-169) and htz1Δ was detectable even in cells with SRB2 intron deleted (srb2Δi). The less truncated variant, prp45(1-330), had a synthetic growth defect with htz1Δ at 16°C, which also persisted in the srb2Δi background. Moreover, htz1Δ enhanced prp45(1-330) dependent pre-mRNA hyper-accumulation of both high and low efficiency splicers, genes ECM33 and COF1, respectively. We conclude that while the expression defects of low expression intron-containing genes contribute to the genetic interactome of prp45(1-169), the genetic interactions between prp45 and htz1 alleles demonstrate the sensitivity of spliceosome assembly, delayed in prp45(1-169), to the chromatin environment.

RPL22 is a tumor suppressor in MSI-high cancers and a key splicing regulator of MDM4

Microsatellite instability high (MSI-H) tumors are malignant tumors that, despite harboring a high mutational burden, often have intact TP53. One of the most frequent mutations in MSI-H tumors is a frameshift mutation in RPL22, a ribosomal protein. Here, we identified RPL22 as a modulator of MDM4 splicing through an alternative splicing switch in exon 6. RPL22 loss increases MDM4 exon 6 inclusion, cell proliferation, and augments resistance to the MDM inhibitor Nutlin-3a. RPL22 represses expression of its paralog, RPL22L1, by mediating the splicing of a cryptic exon corresponding to a truncated transcript. Therefore, damaging mutations in RPL22 drive oncogenic MDM4 induction and reveal a common splicing circuit in MSI-H tumors that may inform therapeutic targeting of the MDM4-p53 axis and oncogenic RPL22L1 induction.

Localization and Functional Roles of Components of the Translation Apparatus in the Eukaryotic Cell Nucleus

Components of the translation apparatus, including ribosomal proteins, have been found in cell nuclei in various organisms. Components of the translation apparatus are involved in various nuclear processes, particularly those associated with genome integrity control and the nuclear stages of gene expression, such as transcription, mRNA processing, and mRNA export. Components of the translation apparatus control intranuclear trafficking; the nuclear import and export of RNA and proteins; and regulate the activity, stability, and functional recruitment of nuclear proteins. The nuclear translocation of these components is often involved in the cell response to stimulation and stress, in addition to playing critical roles in oncogenesis and viral infection. Many components of the translation apparatus are moonlighting proteins, involved in integral cell stress response and coupling of gene expression subprocesses. Thus, this phenomenon represents a significant interest for both basic and applied molecular biology. Here, we provide an overview of the current data regarding the molecular functions of translation factors and ribosomal proteins in the cell nucleus.

Differential expression of duplicated ribosomal protein genes modifies ribosome composition in response to stress

In Saccharomyces cerevisiae, most ribosomal proteins are synthesized from duplicated genes, increasing the potential for ribosome heterogeneity. However, the contribution of these duplicated genes to ribosome production and the mechanism determining their relative expression remain unclear. Here we demonstrate that in most cases, one of the two gene copies generate the bulk of the active ribosomes under normal growth conditions, while the other copy is favored only under stress. To understand the origin of these differences in paralog expression and their contribution to ribosome heterogeneity we used RNA polymerase II ChIP-Seq, RNA-seq, polyribosome association and peptide-based mass-spectrometry to compare their transcription potential, splicing, mRNA abundance, translation potential, protein abundance and incorporation into ribosomes. In normal conditions a post-transcriptional expression hierarchy of the duplicated ribosomal protein genes is the product of the efficient splicing, high stability and efficient translation of the major paralog mRNA. Exposure of the cell to stress modifies the expression ratio of the paralogs by repressing the expression of the major paralog and thus increasing the number of ribosomes carrying the minor paralog. Together the data indicate that duplicated ribosomal protein genes underlie a modular network permitting the modification of ribosome composition in response to changing growth conditions.

Mating-Type-Specific Ribosomal Proteins Control Aspects of Sexual Reproduction in Cryptococcus neoformans

The MAT locus of Cryptococcus neoformans has a bipolar organization characterized by an unusually large structure, spanning over 100 kb. MAT genes have been characterized by functional genetics as being involved in sexual reproduction and virulence. However, classical gene replacement failed to achieve mutants for five MAT genes (RPL22, RPO41, MYO2, PRT1, and RPL39), indicating that they are likely essential. In the present study, targeted gene replacement was performed in a diploid strain for both the α and a alleles of the ribosomal genes RPL22 and RPL39 Mendelian analysis of the progeny confirmed that both RPL22 and RPL39 are essential for viability. Ectopic integration of the RPL22 allele of opposite MAT identity in the heterozygous RPL22a/rpl22αΔ or RPL22α/rpl22aΔ mutant strains failed to complement their essential phenotype. Evidence suggests that this is due to differential expression of the RPL22 genes, and an RNAi-dependent mechanism that contributes to control RPL22a expression. Furthermore, via CRISPR/Cas9 technology, the RPL22 alleles were exchanged in haploid MATα and MATa strains of C. neoformans These RPL22 exchange strains displayed morphological and genetic defects during bilateral mating. These results contribute to elucidating functions of C. neoformans essential mating type genes that may constitute a type of imprinting system to promote inheritance of nuclei of both mating types.