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Position Effect Variegation. 1930- first described. “The mosaic phenotype caused by a chromosomal position effect in which a rearrangement breakpoint displaced the white gene from its normal euchromatic location and placed it in the vicinity of heterochromatin”. Wakimoto Cell 93:321, 1998.

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position effect variegation

Position Effect Variegation

1930- first described

“The mosaic phenotype caused by a chromosomal position effect in which a rearrangement breakpoint displaced the white gene from its normal euchromatic location and placed it in the vicinity of heterochromatin”

Wakimoto Cell 93:321, 1998

position effect variegation pp448 alberts
Position effect variegation- pp448, Alberts

Definition- Translocation of a gene from a euchromatic region to a heterochromatic region resulting in inactivation of nearby heterochromatic genes.

  • Called “heterochromatic spreading”, but is an incomplete definition
position effect variegation1
Position Effect Variegation

The white gene produces

red eyes

Variegated

Wild-type

Dorer and Henikoff Cell 77:993, 1994

pev effect of transgene repeats

Single copy of “mini-white” locus inserted near centromere

  • in null-white fly strain

Heterochromatin

tandem repeat

three repeats

four repeats

PEV- effect of transgene repeats

Inverted repeat

Single copy

Thus, repeat number and orientation affect PEV

slide5

PEV can be suppressed by modifiers

Six-copy

mini-white gene

at 50C in

Su(var)295 flys

controls

models of pev a cis spreading

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

Models of PEVA. Cis-spreading

Note: Histone acetylation effects PEV

1. Cis-spreading, block factor binding

2. Cis-spreading, form repressor complex with factors

Thus, more spreading = more variegation

models of pev a cis spreading1

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

Models of PEVA. Cis-spreading

Problems with cis-spreading model

  • Some hetero-euchromatin rearrangements induce PEV several megabases away
  • PEV is sensitive to interchromosomal interactions
  • Thus, trans-interactions are suggested
models b nuclear compartment model a trans effect model
ModelsB. Nuclear compartment modelA trans-effect model

Evidence in support-

  • Centromeres and most heterochromatin is located at one end of nucleus, telomeres at opposite end
  • Displaced heterochromatic regions interact with other heterochromatic regions
    • prevented by modifiers of PEV
  • However- have not yet correlated measured transcriptional activity and nuclear localization
how does pev occur lets look at telomere position effect

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

How does PEV Occur? Lets look at Telomere Position Effect

RAP1-

  • Telomere Position Effect- Rap1 in complex with SIR proteins (SIR2/SIR3/SIR4) and histones H4 + H3
    • Functions- heterochromatin assembly; recruitment of SIR proteins

Folding-back mechanism

models of pev

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

Models of PEV
  • Over 120 modifiers (enhancers and suppressors) of PEV identified
  • Only some are directly involved
  • HP-1, Su(var)3-7 both co-localize to heterochromatin, interact in yeast two hybrid assay
    • Neither binds DNA
what about genes normally active in heterochromatin

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

What about genes normally active in heterochromatin?
  • Flys have over 20 expressed genes located in heterochromatin
  • >7 of these genes require placement in heterochromatin for normal expression
  • If place into euchromatic region- PEV results!
  • 1/2 of mutations that suppress PEV of euchrom. genes also enhance PEV of heterochrom. genes
what about genes normally active in heterochromatin1

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

What about genes normally active in heterochromatin?

Heterochromatin binding proteins interact with transcription factors to activate transcritpion

or

mediate long-range enhancer-promoter communication

Thus, Rap1p may may have repressor role in euchromatin, activator role in heterochromatin

how is pev maintained

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

How is PEV maintained?
  • No current model is satisfactory
  • Not DNA methylation -(Flys don’t do this)
  • GAGA protein binds to heterochromatin, remains throughout cell cycle
  • DNA must be “tagged” to maintain a given level of PEV during subsequent cell divisions
  • Competition for factors at each cell cycle?
recent result

Advanced Molecular and Cellular Biology Bio4751 Spring 2003

Gary A. Bulla, PhD

Recent result

Mini-white gene

Gal4-responsive Green Fluor. Protein

  • What happens if have two genes (GFP and mini-white) near centromere?

Centromere

Ahmad and Henikoff, Cell 104:839, 2001.

slide15

Ahmad and Henikoff, Cell 104:839, 2001.

GFP in near heterochromatin

GFP in

euchromatin

High GAL4

Low GAL4

Euchrom.

Heterochrom.

  • Observe GFP expression is variegated next to heterochromatin
  • And as increase Gal4, suppress variegation
slide16

What happens to a nearby gene (the mini-white gene)?

Ahmad and Henikoff, Cell 104:839, 2001.

Miniwhite near heterochromatin

Miniwhite in

euchromatin

GAL4?

No

Yes

No

Yes

Thus, GAL4 binding counteracts silencing at

nearby mini-white locus

slide17

Ahmad and Henikoff, Cell 104:839, 2001.

Miniwhite near heterochromatin

Note- GFP on, mini-white off!!

Can GFP and mini-white variegation be uncoupled?

  • Thus, GFP and mini-white silencing can be uncoupled.!
  • Heterochromatic boundary may be within 2 kb of DNA.
  • Heterochomatic spreading in not continuous
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