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Translation initiation from conserved non-AUG codons provides additional layers of regulation and coding capacity

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Ivanov, IP 
Wei, J 
Caster, SZ 
Smith, KM 
Michel, AM 


Neurospora crassa cpc-1 and Saccharomyces cerevisiae GCN4 are homologs specifying transcription activators that drive the transcriptional response to amino acid limitation. The cpc-1 mRNA contains two upstream open reading frames (uORFs) in its >700 nt 5’-leader and its expression is controlled at the level of translation in response to amino acid starvation. We used N. crassa cell-free extracts and obtained data indicating that cpc-1 uORF1 and uORF2 are functionally analogous to GCN4 uORF1 and uORF4 in controlling translation. We also found that the 5’ region upstream of the main coding sequence of the cpc-1 mRNA extends for more than 700 nucleotides without any in-frame stop codon. For 100 cpc-1 homologs from Pezizomycotina and from selected Basidiomycota, 5’ conserved extensions of the CPC1 reading frame are also observed. Multiple non-AUG near-cognate codons (NCCs) in the CPC1 reading frame upstream of uORF2, some deeply conserved, could potentially initiate translation. At least four NCCs initiated translation in vitro. In vivo data were consistent with initiation at NCCs to produce N-terminally extended N. crassa CPC1 isoforms. The pivotal role played by CPC1, combined with its translational regulation by uORFs and NCC utilization, underscore the emerging significance of non-canonical initiation events in controlling gene expression.



neurospora, filamentous fungi, gene regulation, molecular genetics, translational control

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American Society for Microbiology
Wellcome Trust (106207/Z/14/Z)
This work was supported by National Institutes of Health grants (GM068087 to J.C.D., D.B.-P., M.F., and M.S.S. and GM47498 to M.S.S.), the Science Foundation Ireland (grant 08/IN.1/B1889 to J.F.A.), the Wellcome Trust (grant 106207) to A.E.F., and the Texas A&M Institute for Advanced Study (to J.C.D., D.B.-P., and M.S.S.). Funding for the open access charge was from the National Institutes of Health.