title: Epistasis in tomato color mutations involves regulation of phytoene synthase 1 expression by cis-carotenoids.
abstract: Tomato (Solanum lycopersicum) fruit accumulate the red carotenoid pigment lycopene. The recessive mutation yellow-flesh (locus r) in tomato eliminates fruit carotenoids by disrupting the activity of the fruit-specific phytoene synthase (PSY1), the first committed step in the carotenoid biosynthesis pathway. Fruits of the recessive mutation tangerine (t) appear orange due to accumulation of 7,9,7',9'-tetra-cis-lycopene (prolycopene) as a result of a mutation in the carotenoid cis-trans isomerase. It was established 60 y ago that tangerine is epistatic to yellow-flesh. This uncharacteristic epistasis interaction defies a paradigm in biochemical genetics arguing that mutations that disrupt enzymes acting early in a biosynthetic pathway are epistatic to other mutations that block downstream steps in the same pathway. To explain this conundrum, we have investigated the interaction between tangerine and yellow-flesh at the molecular level. Results presented here indicate that allele r(2997) of yellow-flesh eliminates transcription of PSY1 in fruits. In a genetic background of tangerine, transcription of PSY1 is partially restored to a level sufficient for producing phytoene and downstream carotenoids. Our results revealed the molecular mechanism underlying the epistasis of t over r and suggest the involvement of cis-carotenoid metabolites in a feedback regulation of PSY1 gene expression.
authors.affiliation: Department of Genetics, Alexander Silberman Institute of Life Sciences,  Hebrew University of Jerusalem, Jerusalem 91904,  Israel.
authors.forename: David E
authors.initials: DE
authors.lastname: Kachanovsky
authors.forename: Shdema
authors.initials: S
authors.lastname: Filler
authors.forename: Tal
authors.initials: T
authors.lastname: Isaacson
authors.forename: Joseph
authors.initials: J
authors.lastname: Hirschberg
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000096:biosynthesis* / Q000235:genetics; D002338:Carotenoids / Q000235:genetics / Q000378:metabolism*; D004843:Epistasis, Genetic / Q000502:physiology*; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology*; D018506:Gene Expression Regulation, Plant / Q000502:physiology*; D005808:Genes, Recessive; D051232:Geranylgeranyl-Diphosphate Geranylgeranyltransferase; D018551:Solanum lycopersicum / Q000201:enzymology* / Q000235:genetics; D009154:Mutation; D010858:Pigmentation / Q000502:physiology*; D010940:Plant Proteins / Q000096:biosynthesis* / Q000235:genetics
chemical_list: D010940:Plant Proteins; D002338:Carotenoids; D019883:Alkyl and Aryl Transferases; D051232:Geranylgeranyl-Diphosphate Geranylgeranyltransferase