Gene Expression

• overall process by which the information encoded in a gene is converted into an observable phenotype?

  gene expression
• why regulate gene expression?

  adjust to sudden changes, conserve energy, save resources
• code for proteins


  mRNA
• form the basic structure of the ribosome and catalyze protein synthesis


  rRNA
• central to protein synthesis as adaptors between mRNA and amino acids

  tRNA
• splicing of pre-mRNA


  snRNA
• process and chemically modify rRNA

  snoRNA
• telomere synthesis, x chromosome inactivation, protein transport to ER


  other noncoding RNA
• group of genes that are transcribed at the same time, only found in prokaryotes

  operons
• consists of three genes each involved in processing lactose, one of them is the gene for the enzyme b-galactosidase


  lac operon
• hydrolyses lactose into glucose and galactose


  b-galactosidase
• repressor protein is continuously synthesized, it sits on a sequence of dna just in front of the lac operon, the operator site

  repressor protein blocks the promoter site where the rna polymerase settles before it starts transcribing


  lactose is absent
• small amount of allolactose is formed within the bacterial cell, this fits onto the repressor protein at another active site

  this causes the repressor protein to change its shape, it can no longer sit on the operator site, rna polymerase can now reach promoter site


  lactose is present
• rna polymerase can sit on the promoter site but it is unstable and it keeps falling off

  glucose and lactose are present
• activator protein stabilizes rna polymerase, e coli only makes enzymes to metabolise other sugars in the absence of glucose


  glucose absent, lactose present
• summary

  
• bacteria turn groups of genes on and off in response to various environmental signals

  regulatory proteins
• cluster of bacterial genes along with an adjacent promoter that controls the transcription of those genes


  lactose operon
• short region of dna that lies partially within the promoter and that interacts with a regulatory protein that controls the transcription of the operon

  lac operator
• encodes b-galactosidase


  lac z
• encodes lactose permease


  lac y
• carrier protein in the bacterial plasma membrane that moves the sugar into the cells

  lactose permease
• encodes thiogalactoside transacetylase


  lac a
• transfers acetyl group from acetyl-coa to b-galactosidase

  thiogalactoside transacetylase
• produces an mRNA


  lac l
• produces a lac repressor protein


  mRNA
• binds to the operator of the lac operon

  lac repressor protein
• keeps rna polymerase from transcribing the structural genes


  lac repressor
• inhibits transcription of lac operon

  lac repressor
• when present, the lac genes are expressed because allolactose binds to the lac repressor protein and keeps it from binding to the lac operator


  lactose
• binds to an allosteric site on the repressor protein causing a conformational change


  allolactose
• induces the expression of lac genes

  allolactose
• its presence determines whether or not the lac repressor is bound to the operator

  lactose
• releases the repressor to stop additional synthesis of lac mRNA


  enzymes encoded by lac operon
• preferred energy source for e coli

  glucose
• when available, the genes for lactose metabolism are transcribed at low levels


  glucose and lactose
• occurs only when glucose is absent and lactose is present


  maximal transcription of lac operon
• action of cyclic AMP and catabolite activator protein

  maximal transcription of lac operon
• in the absence of inducer, the operon is turned?

  turned off
• exerted by regulatory protein


  control