Abstract
The initial components of ribosomes first appeared more than 3.8 billion years ago during a time when many types of RNAs were evolving. While modern ribosomes are complex molecular machines consisting of rRNAs and proteins, they were assembled during early evolution by the association and joining of small functional RNA units. Introns may have provided the means to ligate many of these pieces together. All four classes of introns (group I, group II, spliceosomal, and archaeal) are present in many rRNA gene loci over a broad phylogenetic range. A survey of rRNA intron sequences across the three major life domains suggests that some of the classes of introns may have diverged from one another within rRNA gene loci. Analyses of rRNA sequences revealed self-splicing group I and group II introns are present in ancestral regions of the SSU (small subunit) and LSU (large subunit), whereas spliceosomal and archaeal introns appeared in sections of the rRNA that evolved later. Most classes of introns increased in number for approximately 1 billion years. However, their frequencies are low in the most recently evolved regions added to the SSU and LSU rRNAs. Furthermore, many of the introns appear to have been in the same locations for billions of years, suggesting an ancient origin for these sequences. In this Perspectives paper, I reviewed and analyzed rRNA intron sequences, locations, structural characteristics, and splicing mechanisms; and suggest that rRNA gene loci may have served as evolutionary nurseries for intron formation and diversification.
Highlights
Pieces of what would become portions of rRNAs and ribosomes first appeared from 3.8 to 4.2 billion years ago, the first rRNAs that contained the PTC, and could synthesize small polypeptides, appeared approximately 3.6 to 3.8 billion years ago (Petrov et al 2014, 2015; Fig. 1). These early ribosomes relied on the delivery of amino acids via tRNAs, which were charged by aminoacyl tRNA synthetases, and short mRNAs
The most parsimonious position for spliceosomal introns is their derivation from group II introns during the origin of Eukarya
Changes in pairing region P9 had mainly detrimental effects on splicing, changes in nucleotides that appeared to have no interactions with other nucleotides (e.g., G41 and G57) had no effect on splicing
Summary
From approximately 4.2–3.8 billion years ago, many innovative types of RNA evolved that played major roles in biology, during an era known as the “RNA world” (Darnell et al 1990; Alberts et al 1998; Gilbert et al 1997; Wekselman et al 2009; Belousoff et al 2010; Rogers 2017; Bada 2013; Huang et al 2013; Iwasa and Marshall 2016) This led to the assembly of the essential components of the central metabolic, evolutionary, and translational functions of ancient cells that still exist in modern cells (Darnell and Doolittle 1986; Gilbert et al 1997; Bujnicki and Rychlewski 2001; Roy et al 2002; Bokov and Steinberg 2009; Rogers 2017; Iwasa and Marshall 2016). From very early in their evolution, ribosomes were (and still are) composed of complexes of structural, enzymatic, kinetic, and binding RNAs, as well as structural proteins (Bokov and Steinberg 2009; Fox 2010; Huang et al 2013; Petrov et al 2014, 2015; Caetano-Annolés et al 2013; Caetano-Annolés and CaetanoAnnolés 2015; Rogers 2017)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
