Abstract
Mitochondrial protein translation that is essential for aerobic energy production includes four essential steps of the mitochondrial ribosome cycle, namely, initiation, elongation, termination of the polypeptide, and ribosome recycling. Translation termination initiates when a stop codon enters the A site of the mitochondrial ribosome where it is recognized by a dedicated peptide release factor (RF). However, RFs and mechanisms involved in translation in plant mitochondria, especially in monocotyledons, remain largely unknown. Here, we identified a crumpled kernel (crk5 allele) mutant, with significantly decreased kernel size, 100-kernel weight, and an embryo-lethal phenotype. The Crk5 allele was isolated using map-based cloning and found to encode a mitochondrial localization RF2a. As it is an ortholog of Arabidopsis mitochondrial RF2a, we named the gene ZmmtRF2a. ZmmtRF2a is missing the 5th–7th exons in the crk5 resulting in deletion of domains containing motifs GGQ and SPF that are essential for release activity of RF, mitochondrial ribosome binding, and stop codon recognition. Western blot and qRT-PCR analyses indicate that the crk5 mutation results in abnormal mitochondrion structure and function. Intriguingly, we observed a feedback loop in the crk5 with up-regulated transcript levels detected for several mitochondrial ribosome and mitochondrial-related components, in particular mitochondrial complexes CI, CIV, and a ribosome assembly related PPR. Together, our data support a crucial role for ZmmtRF2a in regulation of mitochondrial structure and function in maize.
