gene regulation in microbes Flashcards
operon
- group of genes with related functions that share a promoter
- transcribed together producing a single multigenic mRNA
- share binding sites for activators and repressors
e.g. lac operon
promoter
- DNA sequence upstream that RNA polymerase binds to
- initiates transcription
activator
- protein
- binds to DNA near promoter site
- helps RNA polymerase bind to promoter, allowing transcription to begin
- positive regulation
repressor
- protein
- binds to operator site
- prevents RNA polymerase from either binding to the promoter or moving along DNA molecule
- absence allows transcription to begin
- negative regulation
lac operon
- regulates lactose metabolism in prokaryotes
- 3 structural genes
- Z codes for beta galactosidase
- Y codes for permease
- A codes for transcetylase
catabolite repression
- bacteria may have a choice of compound in the environment
- some compounds are easier to metabolise than others
- preferred compound blocks metabolic genes for less preferred compound
allosteric effector
- small molecule
- acts as a sensor
- interacts with activator or repressor at allosteric site
- changes conformation (shape) or protein
- allows it to bind to or be released from DNA
- can activate or inhibit transcription
lac operon, allolactose
- allolactose = allosteric effector
- isomer of lactose produced by cells by the modification of lactose
- binds to lac repressor
- lac repressor no longer able to bind to operator
- acts as an inducer for lac operon in the presence of lactose
lac operon, negative regulation
- LacI gene codes for lac repressor protein
- binds to lac operon and prevents binding of RNA polymerase
- prevents synthesis of enzymes for lactose metabolism
lac operon, lactose metabolism in the presence of glucose
- lactose metabolism has high energy cost as 3 enzymes are required
- glucose metabolism has lower energy cost
- enzymes for lactose metabolism are only produced when lactose is present and glucose is absent
lac operon, lactose metabolism in the presence of glucose, cAMP
- cAMP is present at high concentrations in the absence of glucose
- cAMP binds to catabolic activator protein (CAP) at allosteric site
- allosteric transition allows CAP to bind to CAP binding site
- helps RNA polymerase to bind to promoter
- positive regulation, genes in the lac operon are transcribed in the presence of cAMP (and therefore the absence of glucose)
temperate phages, switching cycles
- lytic cycle, most genes are expressed
- lysogenic cycle, most genes are not expressed
e.g. Bacteriophage λ
switching cycles, Bacteriophage λ operon
OR operator
- overlaps 2 promoters, initiates transcription in opposite directions
- CII activator protein binds to promoter PRE and allows for transcription of cl
Genes
- cI gene codes for λ repressor protein
- cro gene codes for Cro protein
Repressor proteins
- two in competition
- λ repressor protein blocks transcription of cro gene and therefore Cro protein, blocking transcription of lytic genes
- Cro protein, blocks transcription of cro gene and therefore λ repressor protein
Switching cycles in Bacteriophage λ, entering lytic cycle
- bacterial proteases are active when resources are abundant
- bacterial proteases degrade CII activator protein, so it can no longer bind to PRE promoter
- cI can no longer be transcribed
- genes in the other direction can be transcribed, coding for the lytic cycle
Switching cycles in Bacteriophage λ, entering lysogenic cycle
- bacterial proteases are inactive when the bacterial cell is starved and enters dormancy
- CII activator protein can bind to PRE promotor, allowing transcription of cI gene
- cI gene codes for λ repressor protein
- blocks transcription of lytic genes and therefore enters lysogenic cycle
