Noncanonical Translation Initiation of the Arabidopsis Flowering Time and Alternative Polyadenylation Regulator FCA

The RNA binding protein FCA regulates the floral transition and is required for silencing RNAs corresponding to specific noncoding sequences in the Arabidopsis thaliana genome. Through interaction with the canonical RNA 3' processing machinery, FCA affects alternative polyadenylation of many transcripts, including antisense RNAs at the locus encoding the floral repressor FLC. This potential for widespread alteration of gene regulation clearly needs to be tightly regulated, and we have previously shown that FCA expression is autoregulated through poly(A) site choice. Here, we show distinct layers of FCA regulation that involve sequences within the 5' region that regulate noncanonical translation initiation and alter the expression profile. FCA translation in vivo occurs exclusively at a noncanonical CUG codon upstream of the first in-frame AUG. We fully define the upstream flanking sequences essential for its selection, revealing features that distinguish this from other non-AUG start site mechanisms. Bioinformatic analysis identified 10 additional Arabidopsis genes that likely initiate translation at a CUG codon. Our findings reveal further unexpected complexity in the regulation of FCA expression with implications for its roles in regulating flowering time and gene expression and more generally show plant mRNA exceptions to AUG translation initiation.