The RNA effector SERRATE is required for the Arabidopsis polycistronic <i>ctr1-10</i> main open reading frame translation

The T-DNA insertion at the 5'-untranslated region (5'-UTR) of <i>CONSTITUTIVE TRIPLE-RESPONSE1</i> (<i>CTR1</i>) results in an extended 5'-UTR of the <i>ctr1-10</i> mRNA, and the main ORF (mORF) translation requires FRAGILE HISTIDINE TRIAD (FHIT) in the face of upstream open reading frame (uORF) inhibition. The RNA effector SERRATE (SE) was isolated from an enhancer screen for <i>ctr1-10</i>, and CTR1 levels were substantially reduced in <i>se ctr1-10</i> plants. RNA profiling and genetic analyses revealed changes in CTR1 levels independent of pri-miRNA or <i>ctr1-10</i> mRNA processing. Ribosome footprinting analyses revealed an impact on <i>ctr1-10</i> mORF translation efficiency by <i>se</i>. Mutation of the SE-binding cap-binding complex component CAP BINDING PROTEIN20 (CBP20) had effects similar to those of <i>se</i> on CTR1 levels in <i>ctr1-10</i> plants. Sucrose density gradient fractionation of membrane proteins was used to determine the associations of SE with polysome fractions. <i>In situ</i> editing that disrupted an upstream ATG codon proximal to the <i>ctr1-10</i> mORF elevated CTR1 levels in an <i>SE</i>-independent manner. Our study revealed the involvement of SE/CBP20 in the translational regulation of polycistronic <i>ctr1-10</i> mRNAs independent of miRNA biosynthesis and <i>ctr1-10</i> mRNA processing, advancing the knowledge of the heterogeneity of the translation machinery, which plays roles in fine translation control.