Translational control of cell division by elongator

SummaryElongator is required for the synthesis of the mcm5s2 modification found on tRNAs recognizing AA-ending codons. In order to obtain a global picture of the role of Elongator in translation, we used reverse protein arrays to screen the fission yeast proteome for translation defects. Unexpectedly, this revealed that Elongator inactivation mainly affected three specific functional groups including proteins implicated in cell division. The absence of Elongator results in a delay in mitosis onset and cytokinesis defects. We demonstrate that the kinase Cdr2, which is a central regulator of mitosis and cytokinesis, is under translational control by Elongator due to the Lysine codon usage bias of the cdr2 coding sequence. These findings uncover a mechanism by which the codon usage, coupled to tRNA modifications, fundamentally contributes to gene expression and cellular functions

Reciprocal regulation of TORC signaling and tRNA modifications by Elongator enforces nutrient-dependent cell fate

Nutrient availability has a profound impact on cell fate. Upon nitrogen starvation, wild-type fission yeast cells uncouple cell growth from cell division to generate small, round-shaped cells that are competent for sexual differentiation. The TORC1 (TOR complex 1) and TORC2 complexes exert opposite controls on cell growth and cell differentiation, but little is known about how their activity is coordinated. We show that transfer RNA (tRNA) modifications by Elongator are critical for this regulation by promoting the translation of both key components of TORC2 and repressors of TORC1. We further identified the TORC2 pathway as an activator of Elongator by down-regulating a Gsk3 (glycogen synthase kinase 3)-dependent inhibitory phosphorylation of Elongator. Therefore, a feedback control is operating between TOR complex (TORC) signaling and tRNA modification by Elongator to enforce the advancement of mitosis that precedes cell differentiation

A coordinated codon dependent regulation of translation by the Elongator complex

Modifications of transfer RNA (tRNA) are widespread and numerous but their function is not well understood. Particularly, three tRNAs (tRNALysAAA, tRNAGluGAA and tRNAGlnCAA) show universal modifications of the second and fifth carbons of the wobble base. The «Elongator» complex (Elp1-6) is proposed to modify carbon five while the Ctu1-Ctu2 complex is responsible for the thiolation of carbon two. In both cases, the modification is thought to be required to optimize the codon-anticodon interaction. In the fission yeast, the deletion of either complexes results in thermosensitivity associated with growth defects on specific compounds as well as cell cycle and cytokinesis defects. A key step in understanding how these phenotypes result from a tRNA modification defect is to compare the proteome from wild type and mutants. We determined the proteome-wide relative expression level of each S. pombe protein by combining an integrated ORFeome to reverse protein arrays. We identified subsets of proteins whose expression level was altered in the strains lacking either modification. The groups identified can be linked to the phenotypes observed. As a detailed example, the subset of proteins involved in the « mitotic cell cycle transition from G2 to M » has been thoroughly studied. Several genes coding for proteins belonging to this subset show genetic interaction with ctu1 or elp3. One of the more interesting of these genes is cdr2, coding for a protein kinase negatively regulating Wee1 to allow the entry into mitosis. The cell cycle phenotypes associated with the absence of tRNA modifications can be explained by the strong decrease in Cdr2 level observed in the corresponding mutants. Moreover, the ctu1 and elp3 mutant phenotypes mimic most of the reported cdr2 mutant phenotypes. Yet, these phenotypes are aggravated in the triple ctu1 elp3 cdr2 mutant, suggesting that Cdr2 is not the only protein responsible for the phenotypes observed in absence of modification. Why is the translation of the proteins in these subsets – and particularly Cdr2 – hyper sensitive to the absence of mcm5s2U tRNA modification? The answer is found in the codon content of the corresponding genes. Indeed, all the sensitive groups are enriched in AAA codons (by opposition to AAG), which is read by tRNALysUUU, one of the three mcm5s2U modified tRNAs. This link between codon content and sensitivity to the absence of tRNA modification has been demonstrated experimentally via the study of Cdr2. We suppressed the translation defect of Cdr2 by overproducing tRNALysUUU from a plasmid, or by replacing every AAA lysine codon by a AAG-lysine (read by the naturally unmodified tRNALysCUU). Taken all together, these experiments show that the translation of groups of genes enriched in AAA codon is less efficient in the absence of mcm5s2U modification. How is the selection pressure leading to an enrichment of these genes in AAA codons exerted? Why enriching a gene with the lysine AAA codon that is read with low efficiency? These issues are part of the remaining questions. We suggest the possibility of a regulation specific to these groups of genes thanks to their codon bias and the mcm5s2 double modification of tRNAs.(DOCSC03) -- FUNDP, 201