ASH1 mRNA localization in yeast involves multiple secondary structural elements and Ash1 protein translation

Localization of ASH1 mRNA to the distal cortex of daughter but not mother c ells at the end of anaphase is responsible for the two cells' differential mating-type switching during the subsequent cell cycle. This localization d epends on actin filaments and a type V myosin (She1/Myo4). The 3' untransla ted region (3' UTR) of ASH1 mRNA is reportedly capable of directing heterol ogous RNAs to a mother cell's bud [1,2]. Surprisingly, however, its replace ment has little or no effect on the localisation of ASH1 mRNA. We show here that, unlike all other known localization sequences that have been found i n 3' UTRs, all the elements involved in ASH1 mRNA localization are located at least partly within its coding region. A 77 nucleotide region stretching from 7 nucleotides 5' to 67 nucleotides 3' of the stop codon of ASH1 mRNA is sufficient to localize mRNAs to buds; the secondary structure of this re gion, in particular two stems, is important for its localizing activity. Tw o regions entirely within coding sequences, both sufficient to localize gre en fluorescent protein (GFP) mRNA to growing buds, are necessary for ASH1 m RNA localization during anaphase. These three regions can anchor GFP mRNA t o the distal cortex of daughter cells only inefficiently, The tight anchori ng of ASH1 mRNA to the cortex of the daughter cell depends on translation o f the carboxy-terminal sequences of Ash1 protein.