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Current Genetics
https://doi.org/10.1007/s00294-018-0848-2
ORIGINAL ARTICLE
Leaderless mRNAs are circularized in Chlamydomonas reinhardtii
mitochondria
A. Bruce Cahoon
1
· Ali A. Qureshi
1,2
Received: 21 March 2018 / Revised: 27 May 2018 / Accepted: 28 May 2018
© Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
The mitochondrial genome of Chlamydomonas reinhardtii encodes eight protein coding genes transcribed on two polycis-
tronic primary transcripts. The mRNAs are endonucleolytically cleaved from these transcripts directly upstream of their AUG
start codons, creating leaderless mRNAs with 3′ untranslated regions (UTR) comprised of most or all of their downstream
intergenic regions. In this report, we provide evidence that these processed linear mRNAs are circularized, which places
the 3′ UTR upstream of the 5′ start codon, creating a leader sequence ex post facto. The circular mRNAs were found to be
ribosome associate by polysome profiling experiments suggesting they are translated. Sequencing of the 3′–5′ junctions
of the circularized mRNAs found the intra-molecular ligations occurred between fully processed 5′ ends (the start AUG)
and a variable 3′ terminus. For five genes (cob, cox, nd2, nd4, and nd6), some of the 3′ ends maintained an oligonucleotide
addition during ligation, and for two of them, cob and nd6, these 3′ termini were the most commonly recovered sequence.
Previous reports have shown that after cleavage, three untemplated oligonucleotide additions may occur on the 3′ termini
of these mRNAs—adenylation, uridylylation, or cytidylation. These results suggest oligo(U) and oligo(C) additions may be
part of the maturation process since they are maintained in the circular mRNAs. Circular RNAs occur in organisms across
the biological spectrum, but their purpose in some systems, such as organelles (mitochondria and chloroplasts) is unclear.
We hypothesize, that in C. reinhardtii mitochondria it may create a leader sequence to facilitate translation initiation, which
may negate the need for an alternative translation initiation mechanism in this system, as previously speculated. In addition,
circularization may play a protective role against exonucleases, and/or increase translational productivity.
Keywords Mitochondria · Circular RNA · Oligocytidylation · Oligouridylation · Leaderless mRNA
Introduction
Mitochondria originated from a free-living aerobic bacte-
rium that formed an endosymbiotic relationship with a pre-
eukaryotic cell. Over the course of this process, most of
the genes required to maintain mitochondrial integrity have
moved to the nucleus. However, a highly reduced genome
(chondriome) remains that encodes a small proportion of
the proteins necessary for aerobic respiration and mitochon-
drial homeostasis. The expression of these mitochondria-
localized genes is accomplished with a combination of
prokaryotic-like and eukaryotic-like factors unique to this
organelle. One striking feature is the complex array of RNA
maturation processing steps. In the case of photosynthetic
autotrophs (plants and protists) mitochondrial gene expres-
sion requires endonucleolytic cleavage from primary poly-
cistronic transcripts, 5′ and 3′ end processing that includes
untemplated nucleotide additions, exon-splicing, trans-splic-
ing, and degradation mechanisms by enzymes encoded by
the nucleus (Reviewed in Hammani and Giegé 2014). There
is also evidence of retrograde signaling where non-coding
RNAs transcribed in the mitochondria regulate oxidative
phosphorylation-controlling factors encoded in the nucleus
(Reviewed in De Paepe et al. 2018).
Communicated by M. Kupiec.
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s0029 4-018-0848-2) contains
supplementary material, which is available to authorized users.
* A. Bruce Cahoon
abc6c@uvawise.edu
1
Department of Natural Sciences, The University
of Virginia’s College at Wise, Wise, VA 24293, USA
2
Harvard University, Cambridge, MA, USA
@Phil_Robichaud
@deepthiw
@JoseServera