title: Engineering the nifH promoter region and abolishing poly-beta-hydroxybutyrate accumulation in Rhizobium etli enhance nitrogen fixation in symbiosis with Phaseolus vulgaris.
abstract: Rhizobium etli, as well as some other rhizobia, presents nitrogenase reductase (nifH) gene reiterations. Several R. etli strains studied in this laboratory showed a unique organization and contained two complete nifHDK operons (copies a and b) and a truncated nifHD operon (copy c). Expression analysis of lacZ fusion demonstrated that copies a and b in strain CFN42 are transcribed at lower levels than copy c, although this copy has no discernible role during nitrogen fixation. To increase nitrogenase production, we constructed a chimeric nifHDK operon regulated by the strong nifHc promoter sequence and expressed it in symbiosis with the common bean plant (Phaseolus vulgaris), either cloned on a stably inherited plasmid or incorporated into the symbiotic plasmid (pSym). Compared with the wild-type strain, strains with the nitrogenase overexpression construction assayed in greenhouse experiments had, increased nitrogenase activity (58% on average), increased plant weight (32% on average), increased nitrogen content in plants (15% at 32 days postinoculation), and most importantly, higher seed yield (36% on average), higher nitrogen content (25%), and higher nitrogen yield (72% on average) in seeds. Additionally, expression of the chimeric nifHDK operon in a poly-beta-hydroxybutyrate-negative R. etli strain produced an additive effect in enhancing symbiosis. To our knowledge, this is the first report of increased seed yield and nutritional content in the common bean obtained by using only the genetic material already present in Rhizobium.
authors.affiliation: Departamento de Ingenieria Metabólica, Centro de Investigación sobre Fijación de Nitrogeno, Universidad Nacional Autónoma de México, Cuernavaca, Morelos CP62271, Mexico.
authors.forename: Humberto
authors.initials: H
authors.lastname: Peralta
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Mora
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Salazar
authors.forename: Sergio
authors.initials: S
authors.lastname: Encarnación
authors.forename: Rafael
authors.initials: R
authors.lastname: Palacios
authors.forename: Jaime
authors.initials: J
authors.lastname: Mora
mesh_terms: D015964:Gene Expression Regulation, Bacterial; D005818:Genetic Engineering / Q000379:methods; D006885:Hydroxybutyrates / Q000378:metabolism; D009586:Nitrogen Fixation; D009876:Operon; D010088:Oxidoreductases / Q000235:genetics* / Q000378:metabolism; D027805:Phaseolus / Q000382:microbiology*; D011091:Polyesters / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011993:Recombinant Fusion Proteins; D012231:Rhizobium / Q000201:enzymology* / Q000235:genetics; D013559:Symbiosis; D014158:Transcription, Genetic; D000094122:Polyhydroxybutyrates
chemical_list: D006885:Hydroxybutyrates; D010088:Oxidoreductases; D011091:Polyesters; D011993:Recombinant Fusion Proteins; C003182:poly-beta-hydroxybutyrate; D000094122:Polyhydroxybutyrates; C029130:nitrogenase reductase