title: Molecular mechanisms of proline-mediated tolerance to toxic heavy metals in transgenic microalgae.
abstract: Pro has been shown to play an important role in ameliorating environmental stress in plants and microorganisms, including heavy metal stress. Here, we describe the effects of the expression of a mothbean delta(1)-pyrroline-5-carboxylate synthetase (P5CS) gene in the green microalga Chlamydomonas reinhardtii. We show that transgenic algae expressing the mothbean P5CS gene have 80% higher free-Pro levels than wild-type cells, grow more rapidly in toxic Cd concentrations (100 microM), and bind fourfold more Cd than wild-type cells. In addition, Cd-K edge extended x-ray absorption fine structure studies indicated that Cd does not bind to free Pro in transgenic algae with increased Pro levels but is coordinated tetrahedrally by sulfur of phytochelatin. In contrast to P5CS-expressing cells, Cd is coordinated tetrahedrally by two oxygen and two sulfur atoms in wild-type cells. Measurements of reduced/oxidized GSH ratios and analyses of levels of malondialdehyde, a product of the free radical damage of lipids, indicate that free Pro levels are correlated with the GSH redox state and malondialdehyde levels in heavy metal-treated algae. These results suggest that the free Pro likely acts as an antioxidant in Cd-stressed cells. The resulting increased GSH levels facilitate increased phytochelatin synthesis and sequestration of Cd, because GSH-heavy metal adducts are the substrates for phytochelatin synthase.
authors.affiliation: Biophysics Program, Ohio State University, Columbus, Ohio 43210, USA.
authors.forename: Surasak
authors.initials: S
authors.lastname: Siripornadulsil
authors.forename: Samuel
authors.initials: S
authors.lastname: Traina
authors.forename: Desh Pal S
authors.initials: DP
authors.lastname: Verma
authors.forename: Richard T
authors.initials: RT
authors.lastname: Sayre
mesh_terms: D050842:1-Pyrroline-5-Carboxylate Dehydrogenase; D000465:Algorithms; D019881:Aminoacyltransferases / Q000378:metabolism; D000818:Animals; D002104:Cadmium / Q000008:administration & dosage / Q000633:toxicity; D016825:Chlamydomonas reinhardtii / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D005978:Glutathione / Q000378:metabolism; D019216:Metals, Heavy / Q000633:toxicity*; D030781:Organisms, Genetically Modified; D000587:Oxidoreductases Acting on CH-NH Group Donors / Q000235:genetics / Q000378:metabolism; D011392:Proline / Q000378:metabolism*; D013052:Spectrometry, X-Ray Emission
chemical_list: D019216:Metals, Heavy; D002104:Cadmium; D011392:Proline; D050842:1-Pyrroline-5-Carboxylate Dehydrogenase; D000587:Oxidoreductases Acting on CH-NH Group Donors; D019881:Aminoacyltransferases; C093784:glutathione gamma-glutamylcysteinyltransferase; D005978:Glutathione