We discuss here some results which suggest that radically different maxicircle classes coexist within the same kinetoplast. These data, although tentative and incomplete, may provide a new outlook on the kinetoplast genome structure and expression

Alexander A Kolesnikov

DA Maslov

L Simpson

ML Muhich

P Borst

P Sloof

Pavel N Flegontov

PJ Myler

PN Flegontov

SJ Westenberger

ST Lee

SY Lee

English

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BioMed CentralKinetoplastid Biology and Disease
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Radically different maxicircle classes within the same kinetoplast: 
an artefact or a novel feature of the kinetoplast genome?
Pavel N Flegontov† and Alexander A Kolesnikov*†
Address: Department of Molecular Biology, Lomonosov Moscow State University, Vorobjevy Gory 1, Build. 12, 119992 Moscow, Russia
Email: Pavel N Flegontov - flegontov@list.ru; Alexander A Kolesnikov* - sasha@protein.bio.msu.ru
* Corresponding author    †Equal contributors
Abstract
We discuss here some results which suggest that radically different maxicircle classes coexist
within the same kinetoplast. These data, although tentative and incomplete, may provide a new
outlook on the kinetoplast genome structure and expression.
Report
The kinetoplast genome is composed of a large heteroge-
neous population of minicircles and a small number of
maxicircles. The maxicircles contain a conserved coding
region of 16–17 kb and a non-coding "divergent" region
which is highly variable in different strains and species [1-
11]. It is believed that the population of maxicircles
within a single mitochondrion is homogeneous.
The first results contradicting this widely accepted
assumption were obtained by Lee et al. in the early 90-es
[12-16]. Drastic changes of kinetoplast DNA were
observed in strains of Leishmania mexicana amazonensis
selected in vitro for arsenite or tunicamycin resistance, and
these changes were accompanied by amplification of
extrachromosomal DNA in the nucleus [12]. It was dem-
onstrated that a new maxicircle DNA variant appeared in
all drug-resistant strains along with the extrachromo-
somal DNA amplification. This variant was characterized
by an altered pattern of restriction sites [12,13], it had a
radically different sequence of the cytochrome b gene [13]
and also displayed extensive DNA sequence rearrange-
ments in the divergent region [16]. Nevertheless, the
mutant cells demonstrated a normal growth pattern and
apparently retained functioning mitochondria [13].
Moreover, alterations of the minicircle kinetoplast DNA
component ("minicircle dominance switch") were also
observed: the predominant wild-type minicircle class was
replaced by other minicircle classes in the drug-resistant
cells [12-15]. It was hypothesized that the switch of
minicircle dominance might alter editing pathways and
help to generate functional proteins from the altered max-
icircle sequence (for example, of the cytochrome b gene)
[13]. Remarkably, very similar maxicircle variants
appeared in different cloned cell lines independently
selected for arsenite or tunicamycin resistance [13,16].
This fact lead the authors to suppose that there was more
than one maxicircle type in the kinetoplast DNA network
in all studied cell lines, and a minor maxicircle variant
became prevalent after the selection for drug resistance
[13]. Thus, the switch of maxicircle dominance might
accompany the switch of minicircle dominance. This con-
clusion remains controversial because, rather unexpect-
edly, minor maxicircle classes were not detected directly
by PCR or hybridization in the wild-type or drug-resistant
strains and because the results of Lee et al. were never
repeated by another group. However, no alternative
mechanisms underlying the observed drastic changes of
the kinetoplast DNA were proposed.
Published: 18 September 2006
Kinetoplastid Biology and Disease 2006, 5:5 doi:10.1186/1475-9292-5-5
Received: 14 July 2006
Accepted: 18 September 2006
This article is available from: http://www.kinetoplastids.com/content/5/1/5
© 2006 Flegontov and Kolesnikov; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), 
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Page 1 of 2
(page number not for citation purposes)
Kinetoplastid Biology and Disease 2006, 5:5 http://www.kinetoplastids.com/content/5/1/5The new results recently obtained by us [9] on a related
species, Leishmania major, shed an additional light on the
potential phenomenon encountered by Lee et al. with L.
m. amazonensis. We observed that the promastigote-amas-
tigote differentiation in vivo is accompanied by an altera-
tion of the maxicircle divergent region sequence [9]. The
DR structure proved to be drastically different at the pro-
mastigote and amastigote stages. Large-scale sequence
rearrangements and also multiple small-scale insertions/
deletions and single nucleotide substitutions were
observed. Most remarkably, several strains of independent
origin demonstrated an identical rearrangement pattern
suggesting that the hypothetical "switch of maxicircle
dominance" might take place during the differentiation.
However, our results remain tentative, and this work
needs to be repeated in vitro using clonal cell lines.
It is not known but possible that maxicircle genome activ-
ity differs between the amastigote and promastigote
stages. We hypothesized that the structure of the divergent
and/or the coding region in the prevailing maxicircle class
may determine the pattern of genome activity. Thus, a
novel regulatory mechanism may act in the kinetoplast of
Leishmania: a specialized genome for each life cycle stage.
This mechanism remains purely hypothetical, but because
of its novelty and importance it deserves a thorough
experimental verification.
The discussed results suggest that: 1) maxicircles may be
intrinsically heterogeneous (at least in some Leishmania
species); 2) different maxicircle classes may coexist within
a single kinetoplast (since the strains of L. m. amazonensis
used by Lee et al. were clonal); 3) maxicircle classes may
have different divergent region structure and even differ-
ent sequences of some genes; 4) a switch of the predomi-
nant maxicircle class accompanied by shifts in the
abundance of minicircle classes might lead to alteration of
editing patterns; 5) the maxicircle and minicircle dynam-
ics might represent a novel mechanism regulating the
kinetoplast gene expression. These hypotheses provide a
new outlook on the kinetoplast genome expression and,
therefore, deserve rigorous testing.
Conclusion
We hypothesize that a switch of the predominant maxicir-
cle class may take place during the promastigote-amastig-
ote differentiation and under drug pressure. The data
discussed here, although ambiguous and incomplete, sug-
gest that a novel regulatory mechanism may operate in the
kinetoplasts. Subsequent studies will either confirm the
coexistence of radically different maxicircle classes within
the same kinetoplast or provide an alternative explanation
for the observed kinetoplast DNA alterations.
Competing interests
The author(s) declare that they have no competing inter-
ests.
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Radically different maxicircle classes within the same kinetoplast: an artefact or a novel feature of the kinetoplast genome?

Crossref

AM: Comparison of maxicircle DNAs of Leishmania tarentolae and Trypanosoma brucei. Proc Natl Acad Sci USA

Analysis by electron microscopy of the variable segment in the maxi-circle of kinetoplast DNA from Trypanosoma brucei. Biochim Biophys Acta

Chang KP: Characterization of the switch of kinetoplast DNA minicircle dominance during development and reversion of drug resistance in Leishmania. Mol Biochem Parasitol

Chang KP: Transkinetoplastidy: A novel phenomenon involving bulk alterations of mitochondrial kinetoplast DNA of a trypanosomatid protozoan.

Characterization of sequence changes in kinetoplast DNA maxicircles of drug-resistant Leishmania. Mol Biochem Parasitol

Comparison of the maxicircle (mitochondrial) genomes of Leishmania tarentolae and Trypanosoma brucei at the level of nucleotide sequence.

Frasch ACC: Variations in maxi-circle and mini-circle sequences in kinetoplast DNAs from different Trypanosoma brucei strains. Biochim Biophys Acta

GN: Conservative and divergent base sequence regions in the maxicircle kinetoplast DNA of several trypanosomatid flagellates. Mol Biochem Parasitol

KD: Structural organization of maxicircle variable region of Trypanosoma brucei : identification of potential replication origins and topoisomerase II binding sites. Nucleic Acids Res

Kolesnikov AA: Conserved repeats in the kinetoplast maxicircle divergent region of Leishmania sp. and Leptomonas seymouri. Mol Genet Genomics

Kolesnikov AA: The Leishmania major maxicircle divergent region is variable in different isolates and cell types. Mol Biochem Parasitol

NR: Trypanosoma cruzi mitochondrial maxicircles display species- and strain-specific variation and possess a conserved element in the non-coding region.

Selection for arsenite resistance causes reversible changes in minicircle composition and kinetoplast organization in Leishmania mexicana. Mol Cell Biol

Specific A+T-rich repetitive DNA sequences in maxicircles from wildtype Leishmania mexicana amazonensis and variants with DNA amplification. Exp Parasitol

The divergent region of Leishmania tarentolae kinetoplast maxicircle DNA contains a diverse set of repetitive sequences. Nucleic Acids Res

The nucleotide sequence of the variable region in Trypanosoma brucei completes the sequence analysis of the maxicircle component of mitochondrial kinetoplast DNA. Mol Biochem Parasitol

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Radically different maxicircle classes within the same kinetoplast: an artefact or a novel feature of the kinetoplast genome?

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