A non-canonical Puf3p-binding sequence regulates CAT5/COQ7 mRNA under both fermentable and respiratory conditions in budding yeast.

The Saccharomyces cerevisiae uses a highly glycolytic metabolism, if glucose is available, through appropriately suppressing mitochondrial functions except for some of them such as Fe/S cluster biogenesis. Puf3p, a Pumillio family protein, plays a pivotal role in modulating mitochondrial activity, especially during fermentation, by destabilizing its target mRNAs and/or by repressing their translation. Puf3p preferentially binds to 8-nt conserved binding sequences in the 3'-UTR of nuclear-encoded mitochondrial (nc-mitochondrial) mRNAs, leading to broad effects on gene expression under fermentable conditions. To further explore how Puf3p post-transcriptionally regulates nc-mitochondrial mRNAs in response to cell growth conditions, we initially focused on nc-mitochondrial mRNAs known to be enriched in monosomes in a glucose-rich environment. We unexpectedly found that one of the monosome-enriched mRNAs, CAT5/COQ7 mRNA, directly interacts with Puf3p through its non-canonical Puf3p binding sequence, which is generally less considered as a Puf3p binding site. Western blot analysis showed that Puf3p represses translation of Cat5p, regardless of culture in fermentable or respiratory medium. In vitro binding assay confirmed Puf3p's direct interaction with CAT5 mRNA via this non-canonical Puf3p-binding site. Although cat5 mutants of the non-canonical Puf3p-binding site grow normally, Cat5p expression is altered, indicating that CAT5 mRNA is a bona fide Puf3p target with additional regulatory factors acting through this sequence. Unlike other yeast PUF proteins, Puf3p uniquely regulates Cat5p by destabilizing mRNA and repressing translation, shedding new light on an unknown part of the Puf3p regulatory network. Given that pathological variants of human COQ7 lead to CoQ10 deficiency and yeast cat5Δ can be complemented by hCOQ7, our findings may also offer some insights into clinical aspects of COQ7-related disorders.