title: A common transport system for methionine, L-methionine-DL-sulfoximine (MSX), and phosphinothricin (PPT) in the diazotrophic cyanobacterium Nostoc muscorum.
abstract: We present evidence, for the first time, of the occurrence of a transport system common for amino acid methionine, and methionine/glutamate analogues L-methionine-DL-sulfoximine (MSX) and phosphinothricin (PPT) in cyanobacterium Nostoc muscorum. Methionine, which is toxic to cyanobacterium, enhanced its nitrogenase activity at lower concentrations. The cyanobacterium showed a biphasic pattern of methionine uptake activity that was competitively inhibited by the amino acids alanine, isoleucine, leucine, phenylalanine, proline, valine, glutamine, and asparagine. The methionine/glutamate analogue-resistant N. muscorum strains (MSX-R and PPT-R strains) also showed methionine-resistant phenotype accompanied by a drastic decrease in 35S methionine uptake activity. Treatment of protein extracts from these mutant strains with MSX and PPT reduced biosynthetic glutamine synthetase (GS) activity only in vitro and not in vivo. This finding implicated that MSX- and PPT-R phenotypes may have arisen due to a defect in their MSX and PPT transport activity. The simultaneous decrease in methionine uptake activity and in vitro sensitivity toward MSX and PPT of GS protein in MSX- and PPT-R strains indicated that methionine, MSX, and PPT have a common transport system that is shared by other amino acids as well in N. muscorum. Such information can become useful for isolation of methionine-producing cyanobacterial strains.
authors.affiliation: Department of Biochemistry, North-Eastern Hill University, Shillong, 793 022, India, aksingh_nehu@yahoo.co.in.
authors.forename: Arvind Kumar
authors.initials: AK
authors.lastname: Singh
authors.forename: Mayashree B
authors.initials: MB
authors.lastname: Syiem
authors.forename: Rajkumar S
authors.initials: RS
authors.lastname: Singh
authors.forename: Samrat
authors.initials: S
authors.lastname: Adhikari
authors.forename: Amar Nath
authors.initials: AN
authors.lastname: Rai
mesh_terms: D000613:Aminobutyrates / Q000378:metabolism*; D001693:Biological Transport, Active / Q000502:physiology; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism; D008715:Methionine / Q000031:analogs & derivatives / Q000378:metabolism*; D008717:Methionine Sulfoximine / Q000378:metabolism*; D050502:Nostoc muscorum / Q000201:enzymology / Q000378:metabolism*
chemical_list: D000613:Aminobutyrates; D008717:Methionine Sulfoximine; C003121:phosphinothricin; D008715:Methionine; D005974:Glutamate-Ammonia Ligase