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Enhancers

Work at a distance

Orientation independent

Can work in trans

Enhance initiation 31

Enhancer Structure

32

Enhancers Work in Trans

34 Digest Digest + biotin Intact Link

Regulation of and by Host Proteins

Viruses use host and/or virus-specified proteins to regulate gene expression

They either encode and/or bring with them co-activating molecules

Cell type specificity can limit expression

• co-activator molecules can be organ or species specific 35

Viral Transcriptional Activators

Autoregulatory molecules -SV40 Tag, HSV ICP

Some bind DNA - T, EBNA, ICP4, E

Some bind host proteins - HSV VP

Others liberate host TA’s - T, E1A, E 37

Patterns of Regulation

Proteins interact with Pol II to establish regulatory circuits

Positive Autoregulatory Loops -alter the rate of transcription initiation -virus proteins stimulate transcription

Negative Autoregulatory Loops -repress gene expression

Transcriptional Cascade -transcriptional units are activated in a fixed sequence 38

Transcriptional Cascade

Transcription of viral genes in a temporally controlled sequence

Immediate early and early proteins

Transcription of late genes

Ensures coordinated production of DNA genomes and structural proteins, frees template from repressors

Activating proteins can induce transcription of host and viral genes and repress transcription of their own genes 40