1Complementation tests2“Complementation group” equals “Gene”If two mutations failed to complement, they are alleles of the same gene they are allelic to each other they belong to the same complementation groupIf two mutations complements each other,they are alleles of different genesthey are not allelic to each otherthey belong to different complementation groups3Lecture 5: Genetic interactions and epistasisA. Epistasis in a biochemical pathwayB. Epistasis in a regulatory pathwayC. Additive interactionsD. Synergistic interactionsE. SuppressionsRead 14.7 (p632-634); p434-435; 428-429Fig. 14.36; 10.32; 10. 27; 10.284C2redCHIF3HA1A2 BZ1BZ2GLUCOSIDEANTHOCYANINSMt1, Mt2DIHYDROFLAVONOLFLAVAN-3,4-DIOLFLAVANONECHALCONEPeonidin-3-(p-coumaroyl)-rutinoside-5-glucisideepistasis analyses (genetic interactions among different mutations)A. Flavonoid biosynthetic pathway in maizebronze25WT: RedMutations in c2, a1, a2: ColorlessMutations in bz1, bz2: bronzeDouble mutantsC2/a1: colourless-but uninformativebz1/a1: colorless-a1 comes before bz1bz2/a1: colorless-a1 comes before bz2For biosynthetic pathways, the phenotype of the earlier gene in the pathway shows in the double mutant. ie. the earlier-step mutant is epistatic to the late-step mutantDetermine relationship between a1 and c2 by feeding experiment:add flavanone (naringenin): c2+naringenin = reda1+naringenin = colorless6Fig. 7.20Biochemical Pathways4Fig. 7.207B. Regulatory pathwaysSignal A B C D gene expressionPositive action-stimulate next step. Null mutation makes insensitive to signalNegative action-represses next step. Null mutation makes the gene turned on at all time (constitutively)d-: gene expression constitutively oneven in the absence of signalb-: gene expression never turned on even in the presence of the signalb-d- = d- : constitutively onFor regulatory pathways, the phenotype of the later-acting genes shows in the double mutant. ie. the later-acting mutant is epistatic to the earlier-acting mutant58etr1wtethyleneairethyleneairctr1wt ein2EthyleneCTR1 (Kinase)EIN2triple responsectr ein2 :? For regulatory pathways, the phenotype of the later-acting genes shows in the double mutant. ie. the later-acting mutant is epistatic to the earlier-acting mutant9C. Additive pathwaysDouble mutants of dissimilar phenotypes produce a combination of both phenotypesIndicate that the two mutations are in genes acting in separate pathwaysap2-2 (flower abnormal) X gl (no trichome) ap2-2 gl double mutantabnormal flower and no trichome10ap2-2gl111D. Synergistic interactions (enhancement)Two genes may act at the same step of a pathwayOr in parallel or (redundant) pathways12ap1-1ap1-1 cal-1ap1-1 cal-11013E. Suppression Intrgenic suppressorsExtragenic suppressorsAllele-specific suppressionSuppressors are defined classically as mutations thatcorrect the phenotypic defects of another mutationwithout restoring its wild-type sequence. Suppressorsmay be intragenic (affecting the same gene) or they maybe extragenic (affecting a different gene).14Intragenic suppressorsTyrGlyWTTyrGlumut1CysGluE. colitryptophansynthasemut1 mut2Fig. 14.3615Intragenic suppressorFrameshift mutation caused by a single base insertion canbe suppressed by a second mutation that cause a singlebase deletion downstream from the first mutation.See Fig. 10.27-10.28 and p 428-42916Extragenic suppressorsMutation in one gene could correct the effect of a mutation inanother geneNonsense (information) suppressorMutations in genes whose protein products interact17In c. elegans, eight suppressors encode identical tRNAsin which a single C→T substitution changes theanticodon of a tRNATrp gene from 5′-CCA-3′to 5′-CUA-3′. The anticodon change thus allows mutanttRNAs to read the amber codon UAG.Nonsense (information) suppressors18Extragenic suppressorsParticularly useful during genetic analyses, because they oftenidentify additional components of a biological system or
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