distinguish between lac(inducible) & trp(repressible) operons
effector molecule
lac - allolactose, inducer; trp - tryptophan, co-repressor
default state of repressor
lac - active; trp - inactive
default state of operon expression
lac - switched off; trp - switched on
products
lac - protein pdt involved in lac breakdown; trp - protein pdt involved in trp a.a. synthesis
structure
lac - cluster of 3 structural genes coding for enzymes, proteins involved in lac breakdown; trp - cluster of 5 structural genes coding for enzymes in trp biosynthesis pathway
when repressor bind to operator
lac - active form, r&b operator; trp - co-repressor(trp) r&b allosteric site of repressor, repressor r&b operator
define regulatory gene
region of DNA, codes for specific protein pdt that regulates expression of structural genes >
constitutively expressed, i.e always transcribed
suggest advantages of bacteria arranging genes in operons
allow simultaneous regulation of related genes w/ related fn/ involved in same metabolic activity >
genes coding for related proteins/enzymes in single biochemical pathway/related fn/same metabolic activity grouped into operon, easier control >
bacteria able to utilise variety of carbohydrates as respiratory substrate >
operons allow bacteria to adapt, respond to env changes >
operon genes only expressed when required, prevents wastage of energy, resources >
bacteria @ selective advantage
define operon
segment of DNA containing cluster of structural genes, make up single transcription unit, controlled by same promoter, operator >
structural genes code for enzymes/functionally related proteins involved in single metabolic pathway
define structural gene
region of DNA, codes for protein/RNA molecule that forms part of structure/has enzymatic fn
describe main structural features of lac operon
segment of DNA, contains cluster of 3 SG; lacZ, lacY, lacA; makes up single t/c unit controlled by same promoter, operator >
regulatory gene lacl lie upstream operon, codes for lac repressor >
lac operon consists of promoter; RNA pol binding site, operator; lac repressor binding site, CAP binding site; catabolite activator protein binding site beside promoter >
promoter, operator, CAP binding site lie upstream 3 SG >
3 SG codes for enzymes for lac metabolism >
lacZ; B-galactosidase(hydrolyse lactose->glucose+galactose)
lacY; lac permease(membrane protein for transport of lactose into cell)
lac A; lactose transacetylase >
terminator; termination seq, marks end of operon
describe main structural features of trp operon
segment of DNA, contains cluster of 5SG; trpE, trpD, trpC, trpB, trpA; make up single t/c unit controlled by same promoter, operator >
regulatory gene trpR lie upstream operon, codes for trp repressor >
trp operon consists of promoter; RNA pol binding site, operator; trp repressor binding site >
promoter, operator lie upstream 5SG >
5 SG codes for enzymes for trp synthesis >
terminator; termination seq, marks end of operon
how lactose, glucose regulate lac operon expression
lac absent
lac repressor synthesised in active conf; r&b operator >
operon switched off >
RNA pol unable r&b promoter, unable t/c lac operon genes; mRNA not synthesised > BG, LP, LT not produced
lac present
BG convert lac -> allolac >
allolac r&b allosteric site of lac rep, change 3D conf, becomes inactive, unable r&b operator >
operon switched on >
RNA pol able r&b promoter, able t/c lac operon genes; mRNA synthesised >
BG, LP, LT produced
low [glucose], lac present
[cAMP] inversely proportional [glucose] >
CAP activator synthesised inactive >
high [cAMP] r&b allosteric site of CAP, CAP changes 3D conf, becomes active, able r&b CAP binding site >
bends DNA, recruits RNA pol to promoter >
RNA pol binds promoter @ high affinity >
high lac operon t/c -> lactose used as temp substrate
how tryptophan regulate trp operon expression
trp absent
trp repressor synthesised in inactive conf >
operon switched on >
RNA pol able r&b promoter, able t/c trp operon genes; mRNA synthesised, translated into 5 polypeptides >
enzymes that synthesise trp produced, trp synthesised
trp present
trp r&b allosteric site of trp rep, changes 3D conf, becomes active, able r&b operator >
operon switched off >
RNA pol unable r&b promoter, unable t/c trp operon genes; mRNA not synthesised >
enzymes that synthesise trp not produced, trp not synthesised