UAG is a sense codon in several chlorophycean mitochondria

Abstract

The mitochondrial genetic code of those land plants and green algae that have been examined does not deviate from the universal one. A red alga, Chondrus crispus, is the sole reported example throughout the algae that uses a deviant (non-universal) mitochondrial genetic code (UGA=Trp). We have analyzed 366-bp DNA sequences of the gene for mitochondrial cytochrome oxidase subunit I (COXI) from ten chlorophyceaen algae, and detected 3-8 in-frame UAG codons in the sequences of five species. Comparisons of these sequences with those of other algae and land plants have shown that most of the UAG sites in Hydrodictyon reticulatum, Pediastrum boryanum and Tetraedron bitridens correspond to alanine, and those of Coelastrum microporum and Scenedesmus quadricauda to leucine. The three species in which UAG probably codes for alanine are characterized by zoospore formation in asexual reproduction and form a clade in the COXI phylogenetic tree. The two species in which UAG codes for leucine are known to form daughter coenobia and pair in the tree. This is the first report on a deviant mitochondrial genetic code in green algae. Mutational change(s) in the release factor corresponding to UAG would be involved in these code changes. No genetic code deviation has been found in five other species examined.

The Complete Mitochondrial DNA Sequence of Scenedesmus obliquus Reflects an Intermediate Stage in the Evolution of the Green Algal Mitochondrial Genome

“…Although most of the green algal mitochondria (such as of Chlamydomonas spp., Chlorogonium, Prototheca, Nephroselmis) employ the standard genetic code, deviations have been reported in some green algal groups, including other chlorophycean taxa (direct relatives of Scenedesmus) (Hayashi-Ishimaru et al 1996) and Pedinomonas (Turmel et al 1999). Although the type of variation differs among these green algal lineages, all reported deviations involve the use of a termination codon as a sense codon: UGA as tryptophan in Pedinomonas (Turmel et al 1999) and UAG as alanine or leucine in some lineages within the chlorophycean group (Hayashi-Ishimaru et al 1996).…”

“…Although the type of variation differs among these green algal lineages, all reported deviations involve the use of a termination codon as a sense codon: UGA as tryptophan in Pedinomonas (Turmel et al 1999) and UAG as alanine or leucine in some lineages within the chlorophycean group (Hayashi-Ishimaru et al 1996). The Scenedesmus mitochondrion does not use UGA to specify tryptophan; however, like some other chlorophycean algae, it does decode UAG as leucine.…”

Characterisation of a Non-canonical Genetic Code in the Oxymonad Streblomastix strix

“…10,39 Conversely, it is probably very difficult to reassign UAA in eubacteria, since there are two independent factors that recognise this codon. This could, partly, explain why UAA and UAG have never been reassigned together in eubacteria or eubacterial organelles (although UAG has been reassigned alone as either alanine or leucine in the mitochondria of various green algae 40 ).…”

Termination of Protein Synthesis in Mammalian Mitochondria

“…10 -12). Chlorophycean (freshwater green algae) mitochondrial DNA also uses only two stop codons, retaining, in this instance, UAA and UGA, with UAG being recognized by tRNA Leu (14,15). Although there are conflicting opinions, it has been suggested that flatworms and roundworms have diminished their mitochondrial stop codon usage to a single triplet, retaining UAG, with UAA and UGA recoded as tyrosine and tryptophan, respectively (16,17).…”