56
2 Nucleic Acids and Nuclear Proteins
function here as methionine start codons, in addition to AUG [402]. The rRNAs in the kinetoplasts of L. tarentolae, T. brucei and Crithidia fasciculata are 9S (610-612 nt) and 12S
(1141-1150 nt) and are the smallest known. In
spite of their minimum size and unusual composition (83 % A + U), their secondary structure
includes the highly conserved central regions
found in all organisms. The sequences of the
rRNAs of the three species agree at 77-84 % of
positions [97, 401].
The transcripts of the maxi-circles are later
altered by insertion and deletion of uridine residues before they become functional ("RNA editing"). This unique process was discovered
through the uridine residues found in the transcripts that are not encoded by the maxi-circle
DNA. These alterations are often restricted to a
small region of the mRNA and cause, for example, the creation of an initiation codon AUG (in
CO III and URF2 of C. fasciculata) or a shift in a
reading frame (in the CO II gene of C. fasciculata). In contrast, there are 39 extra uridine residues in the cytochrome b mRNA of C. fasciculata
and L. tarentolae. Finally, the CO III transcript in
T. brucei, encoded in a maxi-circle of 450 nt, is
extended to an mRNA of almost 1000 nt by the
insertion of several hundred uridine residues,
such that the final mRNA sequence bears little
resemblance to the corresponding gene [28, 132,
414]. There are two competing ideas about the
mechanism of RNA editing: one proposes that
the edited mRNA is synthesized as a whole [458],
and the other envisages a cyclic process from the
3' to the 5' end involving a family of small transcripts of the maxi- and mini-circles and known as
gRNAs (guide RNAs). Mismatch repair between
the mRNA-gRNA pairs continues until the
mRNA perfectly matches the gRNA and is
released. In L. tarentolae, four maxi-circle-coded
gRNAs are known for maxi-circle genes; the
gRNA for the less exactly edited CO III gene is
encoded by mini-circles [425, 449].
The mini-circles make up about 95 % of the
kDNA. They are uniform in size within an individual network, but may vary species-specifically
between 900 and 2500 bp. The sequences of the
mini-circles show contrasting levels of heterogeneity in different species. In T. brucei there are
several hundred different classes of mini-circle; in
T. equiperdum, on the other hand, little heterogeneity is found. The sequences of mini-circles
vary dramatically, even between closely related
species like Crithidia luciliae and C. fasciculata,
or between the different strains of T. brucei. The
species-specific vanatlons in length and
sequence, together with the sequence heterogeneity, speak for an extremely rapid rate of
evolution of the mini-circles. Their function is,
however, unknown. The open reading frames
(ORPs) of the mini-circles would be sufficient for
proteins of 20-70 amino acids and, in fact, minicircles from C. fasciculata expressed in E. coli
yielded immunoreactive products [124, 396]. The
mini-circles are replicated as free molecules after
release from the network, and the daughter molecules are reincorporated. Thus, the kinetoplast
grows to about double its original size up to the
moment of cell division, when it itself divides.
The two daughter molecules produced during
replication are rather different: one contains the
discontinuously growing H-strand with fragments
of 20-110 nt in length, and the other contains the
continuously growing L-strand. The H-strand is
afterwards partly repaired, whilst the L-strand
becomes nicked; hence, during the reincorporation phase both daughter molecules possess nicks
that are closed on incorporation into the network.
A specific topoisomerase is responsible for the
release of mini-circles (decatenation) from the
network and for the reincorporation of the
daughter molecules (catenation) [372].
References
1. Abad P. et al.: A long interspersed element - the I
factor of Drosophila teissieri - is able to transpose in
different Drosophila species. Proc. Nat. Acad. Sci.
USA 86: 8887-91 (1989)
2. Acampora D. et al.: The human HOX gene family.
Nucleic Acids Res. 17: 10385-402 (1989)
3. Achwal C. w., Iyer C. A. and Chandra H. S.: Immunochemical evidence for the presence of 5mC, 6mA
and 7mG in human, Drosophila and mealybug DNA.
FEBS Letters 158: 353-358 (1983)
4. Adams D. S. et al.: Isolation and partial characterization of U1-U6 small RNAs from Bombyx mori. Biochemistry 24: 117-125 (1985)
5. Adams R. L. P. and Burdon R. H.: Molecular biology
of DNA methylation. Springer, Berlin 1985
6. Adams R. L. P., Leader D. P. and Knowler J. T.: The
biochemistry of nucleic acids, 11th ed. Chapman &
Hall, New York 1992
7. Adams R. L. P.: DNA methylation. The effect of
minor bases on DNA-protein intereactions. Biochern. J. 265: 309-320 (1990)
8. Adelman J. P. et al.: Two mammalian genes transcribed from opposite strands of the same DNA
locus. Science 235: 1514-17 (1987)
9. Aeby P. et al.: Structure and genomic organization of
proretrovirus-like elements partially eliminated from
the somatic genome of Ascaris lumbricoides. Embo J.
5: 3353-60 (1986)
2 Nucleic Acids and Nuclear Proteins
function here as methionine start codons, in addition to AUG [402]. The rRNAs in the kinetoplasts of L. tarentolae, T. brucei and Crithidia fasciculata are 9S (610-612 nt) and 12S
(1141-1150 nt) and are the smallest known. In
spite of their minimum size and unusual composition (83 % A + U), their secondary structure
includes the highly conserved central regions
found in all organisms. The sequences of the
rRNAs of the three species agree at 77-84 % of
positions [97, 401].
The transcripts of the maxi-circles are later
altered by insertion and deletion of uridine residues before they become functional ("RNA editing"). This unique process was discovered
through the uridine residues found in the transcripts that are not encoded by the maxi-circle
DNA. These alterations are often restricted to a
small region of the mRNA and cause, for example, the creation of an initiation codon AUG (in
CO III and URF2 of C. fasciculata) or a shift in a
reading frame (in the CO II gene of C. fasciculata). In contrast, there are 39 extra uridine residues in the cytochrome b mRNA of C. fasciculata
and L. tarentolae. Finally, the CO III transcript in
T. brucei, encoded in a maxi-circle of 450 nt, is
extended to an mRNA of almost 1000 nt by the
insertion of several hundred uridine residues,
such that the final mRNA sequence bears little
resemblance to the corresponding gene [28, 132,
414]. There are two competing ideas about the
mechanism of RNA editing: one proposes that
the edited mRNA is synthesized as a whole [458],
and the other envisages a cyclic process from the
3' to the 5' end involving a family of small transcripts of the maxi- and mini-circles and known as
gRNAs (guide RNAs). Mismatch repair between
the mRNA-gRNA pairs continues until the
mRNA perfectly matches the gRNA and is
released. In L. tarentolae, four maxi-circle-coded
gRNAs are known for maxi-circle genes; the
gRNA for the less exactly edited CO III gene is
encoded by mini-circles [425, 449].
The mini-circles make up about 95 % of the
kDNA. They are uniform in size within an individual network, but may vary species-specifically
between 900 and 2500 bp. The sequences of the
mini-circles show contrasting levels of heterogeneity in different species. In T. brucei there are
several hundred different classes of mini-circle; in
T. equiperdum, on the other hand, little heterogeneity is found. The sequences of mini-circles
vary dramatically, even between closely related
species like Crithidia luciliae and C. fasciculata,
or between the different strains of T. brucei. The
species-specific vanatlons in length and
sequence, together with the sequence heterogeneity, speak for an extremely rapid rate of
evolution of the mini-circles. Their function is,
however, unknown. The open reading frames
(ORPs) of the mini-circles would be sufficient for
proteins of 20-70 amino acids and, in fact, minicircles from C. fasciculata expressed in E. coli
yielded immunoreactive products [124, 396]. The
mini-circles are replicated as free molecules after
release from the network, and the daughter molecules are reincorporated. Thus, the kinetoplast
grows to about double its original size up to the
moment of cell division, when it itself divides.
The two daughter molecules produced during
replication are rather different: one contains the
discontinuously growing H-strand with fragments
of 20-110 nt in length, and the other contains the
continuously growing L-strand. The H-strand is
afterwards partly repaired, whilst the L-strand
becomes nicked; hence, during the reincorporation phase both daughter molecules possess nicks
that are closed on incorporation into the network.
A specific topoisomerase is responsible for the
release of mini-circles (decatenation) from the
network and for the reincorporation of the
daughter molecules (catenation) [372].
References
1. Abad P. et al.: A long interspersed element - the I
factor of Drosophila teissieri - is able to transpose in
different Drosophila species. Proc. Nat. Acad. Sci.
USA 86: 8887-91 (1989)
2. Acampora D. et al.: The human HOX gene family.
Nucleic Acids Res. 17: 10385-402 (1989)
3. Achwal C. w., Iyer C. A. and Chandra H. S.: Immunochemical evidence for the presence of 5mC, 6mA
and 7mG in human, Drosophila and mealybug DNA.
FEBS Letters 158: 353-358 (1983)
4. Adams D. S. et al.: Isolation and partial characterization of U1-U6 small RNAs from Bombyx mori. Biochemistry 24: 117-125 (1985)
5. Adams R. L. P. and Burdon R. H.: Molecular biology
of DNA methylation. Springer, Berlin 1985
6. Adams R. L. P., Leader D. P. and Knowler J. T.: The
biochemistry of nucleic acids, 11th ed. Chapman &
Hall, New York 1992
7. Adams R. L. P.: DNA methylation. The effect of
minor bases on DNA-protein intereactions. Biochern. J. 265: 309-320 (1990)
8. Adelman J. P. et al.: Two mammalian genes transcribed from opposite strands of the same DNA
locus. Science 235: 1514-17 (1987)
9. Aeby P. et al.: Structure and genomic organization of
proretrovirus-like elements partially eliminated from
the somatic genome of Ascaris lumbricoides. Embo J.
5: 3353-60 (1986)