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histriomuscorum
contains three more MDSs (17 total), which simply require
an additional three ld excision operations to assemble the macronuclear gene.
Assembly of the Macronuclear Gene Encoding DNA Polymerase
α
Ten micronuclear genes have been examined in several stichotrichs, and three
are scrambled. The third scrambled gene, the DNA
pol
gene, has the most
complex MDS scrambling pattern. In
Sterkiella nova
the micronuclear gene
contains 45 MDSs, most of which are arranged in an odd/even pattern, as in the
α
α
TP gene, but with inversion of 24 of the MDSs relative to the other 21 MDSs
(Figure 9.27). In addition, MDSs 29-31-33-35-37-39-41-43 and the seven IESs
between these MDSs form a separate group at a different, unknown location in
the genome.
In addition to the DNA
pol
α
gene in
Sterkiella nova
, the structure of the
micronuclear DNA
pol
gene has been determined in
Sterkiella histriomusco-
rum
and
Stylonychia lemnae
. The patterns of scrambling are similar, although
IESs have changed in size and sequence after divergence of the three species
from one another. Migration of IESs has caused modest changes in the sizes
of homologous MDSs, and the MDS number has increased to 48 in
Stylony-
chia lemnae
and to 51 in
Sterkiella histriomuscorum
by insertion of three and
six IESs, respectively. Assembly of this complicated gene structure into its
α
Figure 9.27
A diagram of the micronuclear DNA
pol
α
gene of
Sterkiella nova
. The
gene occurs in two groups of macronuclear destined segments (MDSs) in different
locations in the chromosomal DNA. Because of its length, the larger group is bent into
two lines. It contains an inversion indicated by the long vertical arrow above the top
line of MDSs. Direction of reading of the MDSs is indicated by horizontal arrows in
the MDS blocks. Internal eliminated segments (IESs) are lines between blocks. 5' TAS
and 3' TAS (telomere addition sites) indicate where telomere sequences are added
when the gene is excised from the chromosome. ATG is the start codon for translation,
and TGA is the stop codon. From Prescott and DuBois [9].
