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Abstract

Translational recoding, also known as ribosomal frameshifting, is a
process that causes ribosome slippage along the messenger RNA, thereby
changing the amino acid sequence of the synthesized protein. Whether the
chloroplast employs recoding is unknown. I-iota, a plastome mutant of
Oenothera (evening primrose), carries a single adenine insertion in an
oligoA stretch [11A] of the atpB coding region (encoding a beta-subunit
of the ATP synthase). The mutation is expected to cause synthesis of a
truncated, non-functional protein. We report that a full-length AtpB
protein is detectable in I-iota leaves, suggesting operation of a
recoding mechanism. To characterize the phenomenon, we generated
transplastomic tobacco lines in which the atpB reading frame was altered
by insertions or deletions in the oligoA motif. We observed that
insertion of two adenines was more efficiently corrected than insertion
of a single adenine, or deletion of one or two adenines. We further show
that homopolymeric composition of the oligoA stretch is essential for
recoding, as an additional replacement of AAA lysine codon by AAG
resulted in an albino phenotype. Our work provides evidence for the
operation of translational recoding in chloroplasts. Recoding enables
correction of frameshift mutations and can restore photoautotrophic
growth in the presence of mutation that otherwise would be lethal.