Post-transcriptional regulation through the HO 3'-UTR by Mpt5, a yeast homolog of Pumilio and FBF

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Abstract
Drosophila Pumilio (Pum) and Caenorhabditis elegans FBF bind to the 3'-untranslated region (3'-UTR) of their target mRNAs and repress translation. Pum and FBF are members of a large and evolutionarily conserved protein family, the Puf family, found in Drosophila, C.elegans, humans, and yeasts. Budding yeast, Saccharomyces cerevisiae, has five proteins with conserved Puf motifs: Mpt5/Uth4, Ygl014w, Yll013c, Jsn1, and Ypr042c. Here we report that Mpt5 negatively regulates expression of the HO gene. Loss of MPT5 increased expression of reporter genes integrated into the ho locus, whereas overexpression of MPT5 decreased expression. Repression required the 3'-UTR of HO, which contains a tetranucleotide, UUGU, also found in the binding sites of Pum and FBF. Mutation of UUGU to UACU in the HO 3'-UTR abolished Mpt5-mediated repression. Studies using a three-hybrid assay for RNA binding indicate that Mpt5 binds to the 3'-UTR of HO mRNA containing a UUGU sequence but not a UACU sequence. These observations suggest that the yeast Puf homolog, Mpt5, negatively regulates HO expression post-transcriptionally.