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Figure 9.12
Diagram of the pair of repeats flanking internal eliminated segment (IES)
1inthe
TP gene of
Sterkiella histriomuscorum
. The macronuclear destined seg-
ments (MDSs) are in capital letters, and the IES is in lower case letters. The repeat
sequences are underlined.
β
have been analyzed so far gives an estimate of at least 100,000 IESs, and there-
fore at least 100,000 MDSs, in one complete set (a haploid set) of micronuclear
genes in
Sterkiella nova
. The presence of one or more IESs encrypts a gene
so that it cannot be properly transcribed into a meaningful mRNA molecule.
However, because micronuclear genes are inactive, useless transcription does
not occur.
The vast number of IESs are excised from all the micronuclear genes during
a few hours in the polytene chromosome stage of macronuclear development.
Hence, the number of IESs that are excised is much greater than 100,000, per-
haps as great as several million, depending on the number of DNA molecules in
the polytene chromosomes. Correspondingly, 100,000 to several million MDSs
must be precisely joined, as IESs are excised, to create incipient macronuclear
genes competent for transcription.
An immediate question arises: how does the organism identify the junctions
between MDSs and IESs, cut the DNA precisely at these junctions to remove
the IESs, and ligate the MDSs? At least part of the answer lies in the nucleotide
sequence at the ends of MDSs. For example, MDS 1 in the
TP gene of
Sterkiella
histriomuscorum
diagrammed in Figure 9.11 ends with the nucleotide sequence
5' CAGTA 3' just before it joins IES 1, as shown in Figure 9.12. MDS 2 starts
with the same five nucleotides where the end of IES 1 joins MDS 2.
Thus, the pair of direct repeat sequences of 5 bp may serve as a guiding signal
for joining MDS 1 to MDS 2, simultaneously removing IES 1, as illustrated
in Figure 9.13. A common, well-documented phenomenon in molecular genet-
ics is the parallel alignment of identical DNA sequences, called
homologous
pairing
; usually such paired sequences are present in two separate molecules
(intermolecular pairing). In the case of the MDSs in Figure 9.13, we propose
that the molecule forms a loop so as to align the two copies of CAGTA in
parallel in the same molecule (intramolecular pairing). The aligned sequences
are then recognized by an enzyme that cuts both repeats and rejoins them in a
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