Abstract
Mitochondria carry the remnant of an ancestral bacterial chromosome and express those genes with a system separate and distinct from the nucleus. Mitochondrial genes are transcribed as poly-cistronic primary transcripts which are post-transcriptionally processed to create individual translationally competent mRNAs. Algae post-transcriptional processing has only been explored in Chlamydomonas reinhardtii (Class: Chlorophyceae) and the mature mRNAs are different than higher plants, having no 5′ UnTranslated Regions (UTRs), much shorter and more variable 3′ UTRs and polycytidylated mature mRNAs. In this study, we analyzed transcript termini using circular RT-PCR and PacBio Iso-Seq to survey the 3′ and 5′ UTRs and termini for two green algae, Pediastrum duplex (Class: Chlorophyceae) and Chara vulgaris (Class: Charophyceae). This enabled the comparison of processing in the chlorophyte and charophyte clades of green algae to determine if the differences in mitochondrial mRNA processing pre-date the invasion of land by embryophytes. We report that the 5′ mRNA termini and non-template 3′ termini additions in P. duplex resemble those of C. reinhardtii, suggesting a conservation of mRNA processing among the chlorophyceae. We also report that C. vulgaris mRNA UTRs are much longer than chlorophytic examples, lack polycytidylation, and are polyadenylated similar to embryophytes. This demonstrates that some mitochondrial mRNA processing events diverged with the split between chlorophytic and streptophytic algae.
Highlights
Mitochondria are membrane-bound organelles known for supplying eukaryotic cells with energy through ATP to carry out cellular functions
This study extends the similarities of mitochondrial mRNA processing to include the absence of a 5 UnTranslated Regions (UTRs), despite P. duplex having larger intergenic regions than C. reinhardtii
We present evidence of mitochondrial mRNA processing from two green algae from the chlorophyte and streptophyte lineages with sized circular mitochondrial genomes
Summary
Mitochondria are membrane-bound organelles known for supplying eukaryotic cells with energy through ATP to carry out cellular functions. During the late 20th century, the theory of endosymbiosis became widely accepted and states that an aerobic bacterium was absorbed by, and formed an endosymbiotic relationship with, a pre-eukaryotic cell [2,3] Though it became fully integrated into the Last Eukaryotic Common Ancestor (LECA), the proposed alpha-proteobacterium [4] maintained a portion of its circular genome carrying a conserved set of genes enabling the quick modulation of crucial energy acquisition proteins [5]. This remnant of the bacterial genome is referred to as mtDNA, the mitochondrial genome or chondriome
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