Abstract
Alternative oxidase (AOX) is a mitochondrial inner-membrane oxidase that accepts electrons directly from ubiquinol and reduces oxygen to water without involving cytochrome-linked electron transport chain. It is highly conserved in many non-vertebrate taxa and may protect cells against hypoxia and oxidative stress. We identified two AOX mRNAs in eastern oyster Crassostrea virginica, CvAOXA and CvAOXB, which differ by 170 bp but encode AOXs of the same size. Sequence analyses indicate that CvAOX has 10 exons with a tandem duplication of exon 10, and 3′ alternative splicing using either the first or second exon 10 produces the two variants CvAOXB or CvAOXA, respectively. The second exon 10 in CvAOXA is more conserved across taxa, while the first exon 10 in CvAOXB contains novel mutations surrounding key functional sites. Both variants are expressed in all organs with the expression of CvAOXA higher than that of CvAOXB under normal condition. Under stress by air exposure, CvAOXB showed significantly higher expression than CvAOXA and became the dominant variant. This is the first case of alternative splicing of duplicated exon in a mollusc that produces a novel variant adaptive to stress, highlighting genome’s versatility in generating diversity and phenotypic plasticity.
Highlights
The electron transport chain (ETC) is an essential part of cellular respiration where biochemical nutrients are converted to adenosine triphosphates (ATPs), the main energy source for cellular activities
Alternative oxidase (AOX) in plants plays a critical role in responding to stress, balancing carbon metabolism and electron transport[9, 10], regulating reactive oxygen species (ROS) generated by respiration[2, 11,12,13], and resisting to potent inhibitors of the cytochrome pathway such as cyanide (CN)[14], sulfide[15], nitric oxide (NO)[16], azide[17], and antimycin A18
We identified seven C. virginica AOX (CvAOX) mRNAs by searching published and unpublished EST and transcriptome databases with C. gigas AOX sequence (CGI_10020743)
Summary
The electron transport chain (ETC) is an essential part of cellular respiration where biochemical nutrients are converted to adenosine triphosphates (ATPs), the main energy source for cellular activities. ETC consists of a series of protein complexes that are located on the inner membrane of mitochondria Electrons from donors such as nicotinamide adenine dinucleotide (NADH) and FADH2 are transferred through Complex I (NADH dehydrogenase) and II (succinate dehydrogenase) to ubiquinone, and through Complex III to cytochrome c, which is subsequently oxidized by Complex IV (cytochrome c oxidase), reducing oxygen to water[1, 2] (Fig. 1). AOX in plants plays a critical role in responding to stress, balancing carbon metabolism and electron transport[9, 10], regulating reactive oxygen species (ROS) generated by respiration[2, 11,12,13], and resisting to potent inhibitors of the cytochrome pathway such as cyanide (CN)[14], sulfide[15], nitric oxide (NO)[16], azide[17], and antimycin A18. We report that these variants are produced by alternative splicing of a duplicated exon and may play a role in oyster’s adaptation to environmental stress
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