r-RNA silencing

1
r-RNA silencing

• A. Introduction-
• rRNA produced in nucleoli
• Recall function of rRNA- interact with proteins
  to produce ribosomes for mRNA translation into
  proteins

m
Nucleolar dominance- in interspecific cell
hybrids, only rRNA genes inherited from one
parent are transcribed
2
Background on rRNA silencing
Species A
Species B
Species C

• 1930s- observe loss of secondary
  constrictionsof chromosomes in cell hybrids
• One species is dominant over other
• The constrictions are the location of rRNA arrays

A dominant over B
B dominant over C
A dominant over C
3
Constriction point was called nucleolus organizer
regions NOR
NORs contains 100s to 1000s of copies of rRNA
genes
4
What is mechanisms of rRNA inactivation?

• Two hypotheses

Hypothesis 1- species-specific transcription
factor
Evidence for- human rRNA promoter inactive in
mouse cell free extracts (and vice versa)
but if add human SL1 protein, then murine
extracts will activate human promoter
Human cell extracts
Mouse cell extracts
SL1
Active
Silent
Active
Reporter gene
Human rRNA promoter
Evidence against- Inactivation still occurs
within some crosses of same species. Thus- an
unlikely model
5
What is mechanisms of rRNA inactivation?
Hypothesis 2- enhancer imbalance hypothesis
The organisms with the most enhancers binds up
all available transcription factors hence, it is
only one that expresses rRNA genes
Transcription factor
Species 1
Active
rRNA
rRNA
Species 2
Silent
rRNA
rRNA
Evidence for If introduce many copies of
intergenic regions from species 1 into species
2, then repress species 2 rRNA expression.
Evidence against no differences in number of
binding sites or affinities of binding sites in
plants has been observed
6
Chromosomal context is important in rRNA silencing
Drosophila
X
X
Y
X
NOR ON
NOR OFF
NOR OFF
NOR ON
Dros. m.
Dros. s.
Dros. m.
Dros. s.
Female
Male
Ch6
Ch6
Plants
NOR1 ON
NOR1 ON
NOR1 ON
NOR1 ON
NOR2 OFF
NOR1 ON
Duplication
Ch7
Translocation
NOR2 ON
7
Chromosomal context is important in rRNA silencing
Ch1
Ch1
Ch1
Ch1
Plants
NOR2 ON
NOR1 OFF
NOR2 ON
NOR1 ON
Rye
Wheat
Wheat
Rye
Translocation
8
Are methylation and/or histone acetylation
involved?

• Methylation- controverisal role due to similar
  methylation in active and inactive NORs
• But if demethyalate, activate silent rRNA genes
• Acetylation-
• Acetylase inhibitors activates silent rRNA genes

Perhaps both methylation and acetylation
contribute to silencing rRNA genes