Trypanosoma avium: Large Minicircles in the Kinetoplast DNA

Evolution of RNA editing in trypanosome mitochondria

“…The trypanosomatid mitochondrial or kinetoplast DNA (kDNA) consists of a single highly structured disk-shaped network composed of thousands of catenated minicircles ranging in size from as small as 465 bp in Trypanosoma vivax (21) to as large as 10,000 bp in T. avium (22), and a smaller number of catenated maxicircles ranging in size from 23 to 36 kb. The maxicircles are the homologues of the informational mtDNA molecules found in other eukaryotes and contain 18 tightly clustered protein-coding genes and two rRNA genes; the gene order is conserved in all trypanosomatid species examined.…”

Mitochondrial minicircles in the free-living bodonid Bodo saltans contain two gRNA gene cassettes and are not found in large networks

“…Phylogeny of kinetoplastid minicircles and minicircle networks+ The phylogenetic tree was constructed as described in Materials and Methods, based on the nucleotide and amino acid sequence of a 296 nt cox2 (c)DNA fragment+ The occurrence of kinetoplast DNA networks and gRNAs has been listed, the question mark indicating that the existence of gRNAs has not been formally proved for C. helicis+ The size of the (presumed) gRNA-encoding (mini)circles has been indicated in the right column [*, depending on the strain: 10 kb in strain A1412 and 5+9 kb in strain A493 (Yurchenko et al+, 1999)]+ The significance of the different types of arrows is discussed in the text+ amounts would be released as dimers and trimers+ For other possible linkage numbers (n ϭ 4, n ϭ 6), the number of released monomeric circles predicted to arise would be virtually zero+ Because our preparations contained 55-75% circular monomers, we conclude that the absence of a kDNA network in B. saltans (mt)DNA is not due to its degradation during DNA isolation, but rather to the fact that the minicircles are not organized in a network+ The reason for the existence of minicircle dimers and trimers is unclear+ However, one should realize that comparable amounts of dimers and trimers of circular mt DNAs are also found in mitochondria of mammalian cells, in which they are likely to be (side) products of DNA replication or recombination (Clayton et al+, 1968)+ The mt genetic system of the bodonid B. saltans resembles that of trypanosomatids with respect to maxicircle gene organization, editing patterns of mt mRNAs, and the existence of gRNA-containing minicircles (see Blom et al+, 1998a)+ However, as far as the organization of the kDNA is concerned, B. saltans mt DNA is more similar to other bodonids such as B. caudatus (Hajduk et al+, 1986) and the cryptobiids C. helicis (Lukes et al+, 1998) and T. borreli (Lukes et al+, 1994;Yasuhira & Simpson, 1996)+ In all these species the typical kDNA disc and the catenated kDNA network appear to be missing+ In T. borreli, minicircles are missing altogether and the gRNAs seem to reside on a (single?) 180-kb large circular DNA molecule (Yasuhira & Simpson, 1996)+ In B. caudatus and C. helicis, noncatenated minicircle-like circular DNAs of 10 and 12 (Hajduk et al+, 1986) and 4+2 kb (Lukes et al+, 1998) have been found, but it remains to be demonstrated that these molecules indeed contain gRNA genes+ This makes B. saltans the first organism shown to possess gRNA-containing, noncatenated minicircles (see Fig+ 7)+ It has been proposed that the function of the kDNA network in trypanosomatid flagellates is to prevent the loss of essential minicircle-encoded gRNA genes during kDNA replication (Borst, 1991)+ Although this may be true for trypanosomatids, B. saltans seems to get by perfectly without a network and it remains to be established how the loss of essential minicircles is prevented in this organism+ Possible scenarios could be minicircle redundancy, as proposed for C. helicis, which appears to harbor three times as many (putative) minicircles as C. fasciculata …”

A comparative molecular and 3-dimensional structural investigation into cross-continental and novel avian Trypanosoma spp. in Australia

“…The UMS is the specific binding site for the UMS-binding protein (UMSBP), which is involved in kDNA replication [28] and anti UMSBP from Crithidia fasciculata has been used to detect homologous proteins in other trypanosomatids such as T. cruzi , Leishmania donovani and T. brucei [28–30]. Some trypanosomes isolated from birds such as T. avium possess what appears to be a unique kinetoplast, with unusually large minicircles [26, 31, 32] that make them intrinsically important tool in trying to understand kinetoplastid evolution and biology. As a result of the pleomorphism of avian trypanosomes within species, it has been suggested that in vitro investigations may be more reliable when trying to characterise their structure and understand the differences between species as in vitro studies provide a more stable environment [6, 33].…”