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Lecture 18: Gene regulation and Drosophila sex determination

Post transcriptional regulation

microRNA

splicing

Sex-determination in Drosophila-a case study

Fig. 18.6-18.18; 18.19a; 18.20-24Read: p664-676

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Mechanisms of post transcriptional gene regulationMechanisms of post transcriptional gene regulation

Some genes are regulated Some genes are regulated afterafter transcription. transcription. RNA splicing can regulate expressionRNA splicing can regulate expression.. RNA stability controls amount of gene product.RNA stability controls amount of gene product. mRNA editing can affect biological properties ofmRNA editing can affect biological properties of

protein.protein. Noncoding Noncoding sequences in mRNA can modulatesequences in mRNA can modulate

translation.translation. Protein modification after translation can controlProtein modification after translation can control

gene functiongene function..

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Micro-Micro-RNAs RNAs mediate RNA interference.mediate RNA interference.

2000-2005 2000-2005 –– micro- micro-RNAs RNAs identified and characterizedidentified and characterized RNA interference RNA interference –– trans-acting single stranded trans-acting single stranded

micro-micro-RNAs RNAs that regulate eukaryotic genethat regulate eukaryotic geneexpressionexpression

__ from from introns introns of protein coding transcriptsof protein coding transcripts __ from products of primary transcripts devoid of from products of primary transcripts devoid of

ORFsORFs Number of Number of miRNAs miRNAs may exceed the number ofmay exceed the number of

protein coding genes.protein coding genes.

18-4Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or displayFig. 18.17

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Micro-RNA processing and modes of actionMicro-RNA processing and modes of action

Fig. 18.18a

Immediately after transcription, pri-miRNAs are recognized by Drosha which cropsout pre-miRNA stem loops from larger RNA.Pre-miRNAs undergo active transport from nucleus to cytoplasm where they are recognized byDicer.Dicer reduces the pre-miRNA into a short-lived miRNA:miRNA duplex which is released andpicked up by RISC.

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Two modes of RNA interferenceTwo modes of RNA interference

1. If miRNA and its target mRNA contain perfectly complementary sequences, miRISC cleavesthe mRNA. RNase rapidly degrades cleavage product.

2. If miRNA and its target mRNA have only partial complementarity, cleavage does not occur.miRISC remains bound to its target and represses its movement across ribosomes.

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You Tube movie

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Protein modifications after translation provide aProtein modifications after translation provide afinal level of control over gene function.final level of control over gene function.

Ubiquitination Ubiquitination targetstargetsproteins for degradation.proteins for degradation. Ubiquitin Ubiquitin –– small, highly small, highly

conserved proteinconserved protein Covalently attaches toCovalently attaches to

other proteinsother proteins Ubiquitinized Ubiquitinized proteins areproteins are

marked for degradation bymarked for degradation byproteosomesproteosomes

Fig. 18.19 a

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RNA splicing helps regulate gene expression.RNA splicing helps regulate gene expression.

Fig. 18.16 a

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Sex determination in DrosophilaSex determination in DrosophilaA comprehensive example of gene regulationA comprehensive example of gene regulation

Sex specificSex specifictraits intraits in

DrosophilaDrosophila

Fig. 18.20

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The X:A ratio regulates expressionThe X:A ratio regulates expressionof the Sex lethal (of the Sex lethal (sxlsxl) gene.) gene.

Key factors of sex determination:Key factors of sex determination: Helix-loop-helix proteins encoded by genes onHelix-loop-helix proteins encoded by genes on

the autosomesthe autosomes Denominator elementsDenominator elements

Helix-loop-helix proteins encoded by genes onHelix-loop-helix proteins encoded by genes onthe X chromosomethe X chromosome Numerator elements Numerator elements –– monitor the X:A ratio monitor the X:A ratio

through formation of through formation of homodimers homodimers or or heterodimersheterodimers sisterlesssisterless-A and -A and sisterlesssisterless-B-B

18-13Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or displayFig. 18.21

18-14Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or displayFig. 18.16 b

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SxlSxl triggers a cascade of splicing.triggers a cascade of splicing.

SxlSxl influences splicing of influences splicing of RNAs RNAs in other genes.in other genes. e.g., e.g., transformertransformer ( (tratra))

Presence of Presence of Sxl Sxl produces functional protein.produces functional protein. Absence of Absence of SxlSxl results in nonfunctional protein.results in nonfunctional protein.

Fig. 18.22 a

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Cascade of splicing continuesCascade of splicing continues e.g., e.g., doublesex doublesex ((dsxdsx))

Tra Tra protein synthesized in females along with Tra2 proteinprotein synthesized in females along with Tra2 protein(produced in males and females) influences splicing of (produced in males and females) influences splicing of dsxdsx..

Females - Produces female specific Females - Produces female specific DsxDsx-F protein-F protein Males Males –– No No Tra Tra protein and splicing of protein and splicing of DsxDsx produces produces DsxDsx-M-M

proteinprotein

Fig. 18.22 b

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DsxDsx-F and -F and DsxDsx-M are transcription factors that determine-M are transcription factors that determinesomatic sexual characteristics.somatic sexual characteristics.

Alternative forms of Alternative forms of Dsx Dsx bind to YP1 enhancer, but have oppositebind to YP1 enhancer, but have oppositeeffects of expression on YP1 gene.effects of expression on YP1 gene. DsxDsx-F is a transcriptional activator.-F is a transcriptional activator. DsxDsx-M is a transcriptional repressor.-M is a transcriptional repressor.

Fig. 18.23

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Tra Tra and Tra-2 proteins also helpand Tra-2 proteins also helpregulate the expression of regulate the expression of Fruitless.Fruitless.

Primary Primary fru fru mRNA transcript made in both sexesmRNA transcript made in both sexes Presence of Presence of tra tra protein in females causes alternative splicingprotein in females causes alternative splicing

encoding encoding frufru-F.-F. Absence of Absence of tra tra protein in males produces protein in males produces frufru-M.-M.

Fig. 18.24

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