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r-RNA silencing

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.

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r-RNA silencing

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  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 constrictions”of 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 Plants Ch6 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

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