title: Aspergillus nidulans genes encoding reverse transsulfuration enzymes belong to homocysteine regulon.
abstract: Homocysteine is an intermediate in methionine synthesis in Aspergillus nidulans, but it can also be converted to cysteine by the reverse transsulfuration pathway involving cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL). Because homocysteine is toxic to the cell at high concentrations, this pathway also functions as a means of removal of its excess. We found that the transcription of the mecA and mecB genes encoding CBS and CGL was upregulated by excess of homocysteine as well as by shortage of cysteine. Homocysteine induced transcription of both genes when added to the growth medium or overproduced in a regulatory mutant. The derepressing effect of cysteine shortage was observed in some mutants and in the wild-type strain during sulfur starvation. An increase in the level of mecA or mecB transcript roughly parallel with the elevation of the respective enzyme activity. On the basis of the mode of mecA and mecB regulation by homocysteine, these genes may be classified in a group of genes upregulated directly or indirectly by this amino acid. We call this group of genes the "homocysteine regulon".
authors.affiliation: Institute of Biochemistry and Biophysics, Polish Academy of Sciences, 5A Pawińskiego Str, 02-106, Warsaw, Poland. marsi@ibb.waw.pl
authors.forename: Marzena
authors.initials: M
authors.lastname: Sieńko
authors.forename: Renata
authors.initials: R
authors.lastname: Natorff
authors.forename: Sylwia
authors.initials: S
authors.lastname: Owczarek
authors.forename: Igor
authors.initials: I
authors.lastname: Olewiecki
authors.forename: Andrzej
authors.initials: A
authors.lastname: Paszewski
mesh_terms: D000595:Amino Acid Sequence; D001233:Aspergillus nidulans / Q000201:enzymology / Q000235:genetics* / Q000254:growth & development; D003001:Cloning, Molecular; D003470:Culture Media; D003541:Cystathionine beta-Synthase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D005800:Genes, Fungal; D006710:Homocysteine / Q000378:metabolism*; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid; D013455:Sulfur / Q000378:metabolism*; D014158:Transcription, Genetic
chemical_list: D003470:Culture Media; D006710:Homocysteine; D013455:Sulfur; D003541:Cystathionine beta-Synthase
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