Within the order Kinetoplastida there are two major subgroups. One subgroup is the trypanosomatids, and the other is the bodonids and related cryptobiids. The trypanosomatids include the trypanosomes and several other species, including Leishmania, Crithidia, Herpetomonas and Blastocrithidia. Within both of these subgroups, different species undergo different degrees of Uridine insertion/deletion, but uridine insertion/deletion occurs in all of them. (see figure 1) (Maslov, et al. 1994)

There are two degrees of RNA editing that are seen in trypanosomatids, pan-editing and 5'-editing. Pan-editing is where the entire gene is edited, while 5'-editing is where only the 5' end of the mRNA molecule is edited. Some of the genes found in the mitochondrial DNA are broken down into two domains. In this case, when pan-editing occurs, both domains are edited, but when 5'-editing occurs, only one domain is edited. (Simpson et al. 2000)

In some of the species, such as T. brucei, pan-editing occurs in almost all genes in the maxicircle, while in others, such as L. tarentolae, 5'-editing occurs in almost all genes. A third species, B. culicis, undergoes both 5'-editing and non-editing. In this species, the gene MURF4 undergoes 5'-editing, while the gene COIII is non-edited.(see figure 1) This is interesting, because this is the first time that a gene that undergoes pan-editing in one species undergoes no editing in another. (Maslov, et al. 1994)

It is thought that pan-editing was present in ancestral organisms, and that then 5' editing, or non-editing, replaced pan-editing in several species. This is because pan-editing has been found in early diverging branches, suggesting that it is the ancestral trait. There are different mechanisms for why 5'-editing replaced pan-editing. One of these mechanisms is the thought that partially edited RNAs underwent reverse-transcription and were made into cDNAs that then were incorporated into maxicircles and replaced the pan-edited genes. The reason why pan-editing was replaced with reverse transcribed 5' edited genes was because minicircles and guideRNAs, or gRNAs, were lost during replication and segregation in certain species. Minicircles replicate when the cell replicates itself during the lifecycle, and then segregate at random, and so some daughter cells may not contain mitochondria that have certain minicircles that encode for gRNAs. (Maslov, et al. 1994)

T. brucei has retained pan-editing most probably due to the fact that it uses uridine insertion or deletion as a translational control mechanism. The fact that T. brucei has a large amount of gRNAs that overlap each other, instead of a few gRNAs that don't overlap as much, as in other species, could be to prevent the loss of the gRNAs during the creation of daughter cells, or it could signify that this is the primitive state. (Maslov, et al. 1994) It is thought that the fact that 5'-editing was also retained, instead of being replaced by non-edited genes, was probably due to translational control, to be able to produce a methionine amino acid to begin translation with. (Simpson et al. 2000)

It is thought that RNA editing could have first evolved in trypanosomes due to a gene duplication mechanism, or it could be carried over from the original mitochondrion that was taken up by eukaryotes. (Maslov, et al. 1994) U insertion/deletion most probably arose in the early bodonid-kinetoplastid lineage, after the euglenoids diverged. This is thought, because RNA editing, and gRNAs are found in genome of some of the species from the bodonid subgroup, which is the most primitive subgroup found in the kinetoplastid order. The next step in the evolution of trypanosomatids is thought to be the catenation, or connection of the kinetoplastid DNA, or kDNA, into circles, megacircles as in T. borreli, or a network of circles as in trypanosomatids. (Simpson et al. 2000)

It appears that RNA editing has been found to appear during evolution several different times in different organisms. It is thought that the reason RNA editing has evolved is to correct the errors in the mitochondrial DNA so that the mitochondria can function correctly to help the cell run correctly. It also may have evolved to modify the genotype through the RNA. (Simpson et al. 2000)

Works Cited:

Maslov, D., Avila, H., Lake, J., and Simpson, L. (1994). Evolution of RNA editing in kinetoplastid protozoa. Nature 368: 345-348.

Simpson, L., Thiemann, O., Savill, N., Alfonzo, J., and Maslov, D. (2000). Evolution of RNA editing in trypanosome mitochondria. Proceedings of the National Academy of Sciences USA 97: 6986-6993.

Created by Erin Sargent as part of a biology senior seminar at Earlham College

Last updated:April 24, 2003