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 the β-subunit of the ATP synthase). The mutation is expected to cause synthesis of a truncated, nonfunctional 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 a mutation that otherwise would be lethal.
Highlights
Plastids are plant organelles that harbor their own genome and are essential for many metabolic pathways, including photosynthesis and de novo synthesis of amino acids, nucleotides and fatty acids (Jarvis and López-Juez, 2013)
Plastome mutants are an important tool for the analysis of non-Mendelian inheritance, the study of photosynthesis, and the analysis of the mechanisms of chloroplast gene expression (Greiner, 2012; Sobanski et al, 2019)
Efficient recoding requires a homopolymeric sequence Recoding based on transcriptional slippage relies on homopolymeric sequences (Baranov et al, 2005), while PRF can occur on either homopolymeric or heteropolymeric slip sites (Baranov et al, 2011)
Summary
Plastids (chloroplasts) are plant organelles that harbor their own genome (plastome) and are essential for many metabolic pathways, including photosynthesis and de novo synthesis of amino acids, nucleotides and fatty acids (Jarvis and López-Juez, 2013). Extant plastid genomes are small, and contain only coding information for approximately 120 genes in green plants (Bock, 2007; Barkan, 2011). The majority of these plastid-encoded genes are crucial for plant viability and required for photoautotrophic growth, including, for example, genes encoding the large subunit of RuBisCO, the reaction center subunits of the two photosystems, and subunits of the cytochrome b6f complex and the adenosine triphosphate (ATP) synthase. Due to its cyanobacterial ancestry, the chloroplast gene expression machinery is largely of bacterial type (Peled-Zehavi, 2007; Zoschke and Bock, 2018)
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