Regulation of Gene Expression    pg 701-30 ; p 741-21

    Permitting or inhibiting transcription - gene "off" or "on"
    Catabolic vs anabolic pathways

Prokaryotes - adaptive enzyme synthesis

   Operon model - Jacob and Monod
         Single regulatory unit
         Includes - structural genes for pathway and regulatory genes
         Polycistronic mRNA

Two types of negative control - Repressor protein turns off transcription

        Repressor protein  
            Binds to operator which prevents RNA polymerase from binding to promotor
	Regulatory gene outside operon codes for repressor
	Repressor is an allosteric protein 

     Inducible operon - default condition is off, must be turned on
         Common to catabolic  pathways 
         Example: lac operon  [Fig 21-4] 
             Regulator gene - lacl  (outside operon) - codes for repressor protein
             Operator and Promoter
             Structural genes for Beta-gal, permease, transacetylase
             Effector - allolactose - inactivates repressor

            Presence of lactose induces transcription - gene turned on

     Repressible operon - must be turned off
          Common to anabolic  pathways 
          Example: Trp operon [Fig 21- 6]
             Operator, Promoter and Leader seq
             Structural genes for enzymes in tryp pathway
	 Regulator gene - trpR  (outside operon) - codes for repressor protein
             Effector - corepressor, tryptophan (product) - activates repressor
            
Also (Ch 19) sigma factors
-------------
Eukaryotes - multiple levels of control [Fig 21-11]
                       the single cistron model

   Accessibility of genes
       Nucleus
       Generalized repression of chromosomal regions - heterochromatin
           nucleosome  (Ch 16)
       
   Transcriptional Control - most often at initiation
       Different RNA polymerases
       Common organization of eukar. gene  [Fig 21-21]
           Basal transcription factors include a common DNA binding protein
             (TFIID)and other proteins forming complex at core promotor permitting 
             low levels of transcription 
           Proximal regulatory elements (part of DNA)upstream - sites for interaction
              with other regulatory transcription  factors [prots]

      Combinatorial model of gene regulation  [Fig 21-23, 24]
          Regulatory sequences can be anywhere - not like prokaryotes

          Regulatory proteins [activators] bind to enhancer region of DNA -
           enhanceosome
              DNA bends to position enhanceosome near core promotor
              Reg prots of enhcanceosome bind to coactivators on TFIID facilitating
                its binding to the promotor
              Other TFs and RNA polymerase form complex and transcription ensues

          Silencers are known but poorly understood

Post-transcriptional regulation
      Splicing Ch 20, etc
      Functional conformation

mRNA half life[Fig 21-34]

Post-translational regulation 

Protein degradation by proteasome  [Fig 21-36]