Recognition of a weak-context upstream-AUG codon in the AtMHX gene is facilitated by the secondary structure of the 5′ untranslated region

While the importance of the sequence-context for recognition of initiator-AUG codons is well established, there are only a few examples of the control of gene expression by the structural context of AUG codons. Translation of the Arabidopsis metal transporter AtMHX is substantially repressed due to efficient recognition of an upstream-AUG (uAUG) codon included in its 5′ untranslated region (5′UTR). This efficient recognition was surprising, because the uAUG codon of AtMHX has a weak sequence context. We investigated the possibility that the secondary structure of this 5′UTR facilitates the efficient recognition of this weak-context uAUG codon. Mutations designed to disrupt the pairing of a stem and loop structure, in which the uAUG codon is localized, reduced recognition of this codon, while complementary mutation restored a great extent of the recognition. The secondary structure did not have an independent effect on downstream translation, but its impact was related to the presence of the uAUG codon. The structural modification was able to affect the efficiency of downstream translation only when the sequence context of the uAUG codon was weak. These data indicate that AtMHX translation is governed by the impact of the secondary structure of its 5′UTR on recognition of the weak-context uAUG codon.