Position Effect Variegation PEV

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Position Effect Variegation (PEV)
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PEV Model

• In 1939, Schultz offered the first model to
  explain the position effect variegation results
  of Morgans X-ray induced mutants
• Observations showed that whenever a gene more
  distant from a breakpoint showed a mutant
  phenotype (white), the gene closer did also
  (Notch).
• An inactivation process spreads from the
  heterochromatic breakpoint along the chromosome.
• Visually the area became darkened, disarranged
  and sometimes completely heterochromatic.
• The oozing model was proposed in the late
  1980s depends on heterochromatic-specific
  proteins propagating continuously along the
  chromosome and blocking access to transcription
  factors.

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Paul B. Talbert and Steven Henikoff (2000) A
Reexamination of Spreading of Position-Effect
Variegation in the white-roughest Region of
Drosophila melanogaster . Genetics 154, 259-272.
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Figure 1. Schematic diagrams of the wm4, wmMc,
and w51b inversions. The top line shows a
magnified view of the region around the w gene.
Other solid lines represent the X chromosome,
which is not drawn to scale. Dashed lines
indicate the location of type I and 359-bp
repeats. Open boxes, ribosomal DNA repeats
filled boxes, other heterochromatin vertical
arrows, inversion breakpoints horizontal arrow,
the w transcription unit. Data from APPELS and
HILLIKER 1982 and TARTOF et al. 1984 .
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Figure 5. The eye of a typical 4LMcR/0 male with
mostly rough ommatidia. The arrowheads indicate
pigmented (w) cells within the rough area.
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Figure 3. Variegated eye phenotypes of X/0 males.
Note the rough patches in the eyes of the McT/0,
4LMcR/0, and 51b/0 flies (arrowheads).
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Findings

• Rearrangement-linked differences in spreading are
  not explained by the oozing model white close
  to pericentric heterochromatin can be expressed
  whereas white further away may not.
• Spreading appears to be discontinuous often the
  white gene seems to be skipped.
• Increased total heterochromatin correlates with
  trans-acting suppression.
• The results are consistent with the coalescence
  model of spreading.
• Heterochromatin formation is nucleated by
  pairing structures formed between repeated
  sequences.

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Telomeric PEV
Diane E. Cryderman, Eric J. Morris, Harald
Biessmann, Sarah C.R. Elgin and Lori L. Wallrath
(1999) Silencing at Drosophila telomeres nuclear
organization and chromatin structure play
critical roles. EMBO J. 18, 3724-3735.
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Outline

• Transgenes inserted into telomeric regions of
  Drosophila chromosomes display PEV.
• Telomeric transgenes on chromosomes 2 and 3 are
  flanked by telomeric associated sequences, on
  chromosome 4 by repetitious transposable
  elements.
• Telomeric PEV on chromosomes 2 and 3 is
  suppressed by mutations in Su(z)2, but not by
  mutations in Su(var)2-5 (encoding HP1).
• Fourth chromosome PEV is therefore
  mechanistically similar to that operating in
  pericentric regions whereas PEV at chromosomes 2
  and 3 is mechanistically different.

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Fig. 1.   Eye phenotypes of flies with a
telomeric hsp70-white transgene in different
genetic backgrounds. Transgenes near the
telomeric region of the fourth chromosome, such
as in stock 39C-72, show suppression of PEV when
heterozygous for a mutation in the gene encoding
HP1, but not when heterozygous for a mutation in
Su(z)2. Transgenes inserted near the telomeric
regions of the second and third chromosomes, such
as in stocks 39C-5 and 118E-26, show suppression
of PEV when heterozygous for a mutation in
Su(z)2, but not when heterozygous for a mutation
in the gene encoding HP1. Top line names of
stocks and locations of P-element inserts. Left
column genotypes of the flies. Abbreviations T
4, telomere of the fourth chromosome T 2L,
telomere of the left arm of chromosome two T 3R,
telomere of the right arm of the third
chromosome , wild-type chromosome Cy,
inversion balancer chromosome carrying the Curly
mutation.
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Is the PEV due to Location, Adjoining Sequences
or Both?

• Induced telomere swapping through X-rays.
• Mapped location of the translocated P element
  containing the white gene and associated
  chromosomal region.
• Selected (i) translocations of telomere 4 to left
  and right ends of chromosome 2, (ii) to the
  pericentric region of chromosome 3 and (iii)
  translocation of telomere 2 to telomere 4.
• Examined variegation and effects of Su(var)2-5
  and Su(z)2 on suppression of variegation.

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Fig. 3.   Cytology of the fourth chromosome
translocations. Cytological regions 102A-F of the
fourth chromosome are translocated onto 2L (at
cytological region 25A) in stock X-2. Cytological
regions 102A-F of the fourth chromosome are
translocated onto 2R (at cytological region
56D-F) in stock X-4.
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Fig. 4.   Eye phenotypes of fourth chromosome
rearrangements in different genetic backgrounds.
(A) Diagrams of the relevant chromosomes are
shown at the top with a circle representing the
centromere and the red arrowhead labeled with a
P' representing the P-element. The fourth
chromosome is represented in green, the second
and third chromosomes in blue. Other designations
are as for Figure 1. Translocation of 1 Mb of
the distal portion of a fourth chromosome
carrying the P-element at its telomere to the
distal end of chromosome two stocks labeled
T(24) causes a reduction in the degree of
silencing of the white transgene. Transposition
of approximately the same fourth chromosome
fragment to a proximal region of the third
chromosome stock Tp(43) does not significantly
alter the phenotype. The PEV observed in all of
these cases is suppressed when the flies are
heterozygous for a mutation in the gene encoding
HP1, Su(var)2-502. (B) Diagram of the relevant
chromosomes at the top. Translocation of a distal
region of chromosome two, carrying the white
transgene at the telomere, onto the distal region
of the fourth chromosome stock T(24) does not
alter the degree of PEV. In both cases shown, the
PEV is suppressed when the flies are heterozygous
for mutations in Su(z)2, but is not affected by
mutations in the gene encoding HP1 (latter not
shown).
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Summary

• Translocation of 1 Mb of the distal portion of
  chromosome 4 to the distal end of chromosome 2
  causes a reduction in the degree of silencing of
  the white transgene (regardless of which end of
  chromosome 2).
• Translocation of the same fragment to the
  proximal pericentric region of chromosome 3 does
  not alter the phenotype.
• PEV in these instances is suppressed in
  Su(var)2-5 heterozygotes but not in Su(z)2
  heterozygotes .
• Translocation of the distal portion of chromosome
  2 to to the distal end of chromosome 4 does not
  alter the degree of PEV.
• PEV in this instance is suppressed in Su(z)2
  heterozygotes but not in Su(var)2-5 heterozygotes.

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The Model
Fig. 7.   Model for nuclear compartments of
silencing components. (A) HP1-sensitive chromatin
domains are affected by nuclear position. The
location of a telomeric transgene within an
HP1-sensitive domain (black) is marked by a red
arrowhead. This transgene shows strong PEV due to
its proximity to centric heterochromatin, which
contains many binding sites for silencing
components such as HP1 (small shaded ovals left
cartoon). Numerous binding sites generate an
increased effective concentration of silencing
factors within this region of the nucleus. Upon
translocation to the telomeric region of a longer
chromosome, the translocated material is
mislocalized to a different compartment within
the nucleus (right cartoon). An association of
the translocated region with silencing factors is
maintained due to the cotranslocation of
cis-acting sequences however, the total number
of binding sites occupied is presumed to decrease
due to a lower effective concentration of these
silencing factors in this region of the nucleus.
This results in increased gene expression.
(B) HP1-insensitive telomeric chromatin domains
are unaffected by nuclear position. The location
of a telomeric transgene within an
HP1-insensitive domain (green) is marked by an
arrowhead (left cartoon). Upon translocation to
the telomeric region of a shorter chromosome, the
translocated material is mislocalized within the
nucleus into a compartment that has a high
concentration of HP1 and associated silencing
factors (right cartoon). The translocated
telomeric region retains its local chromatin
structure, which does not depend on HP1
therefore, gene expression remains unchanged.
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Conclusion

• Both nuclear organisation and chromatin structure
  have an impact on PEV
• The nucleus must be composed of sub-regions that
  differ in their capacity to support gene
  expression.

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