Francine Casse

Functional analysis of AtHKT1 in Arabidopsis shows that Na+ recirculation by the phloem is crucial for salt tolerance

Two allelic recessive mutations of Arabidopsis, sas2‐1 and sas2‐2, were identified as inducing sodium overaccumulation in shoots. The sas2 locus was found (by positional cloning) to correspond to the AtHKT1 gene. Expression in Xenopus oocytes revealed that the sas2‐1 mutation did not affect the ionic selectivity of the transporter but strongly reduced the macro scopic (whole oocyte current) transport activity. In Arabidopsis, expression of AtHKT1 was shown to be restricted to the phloem tissues in all organs. The sas2‐1 mutation strongly decreased Na+ concentration in the phloem sap. It led to Na+ overaccumulation in every aerial organ (except the stem), but to Na+ underaccumulation in roots. The sas2 plants displayed increased sensitivity to NaCl, with reduced growth and even death under moderate salinity. The whole set of data indicates that AtHKT1 is involved in Na+ recirculation from shoots to roots, probably by mediating Na+ loading into the phloem sap in shoots and unloading in roots, this recirculation removing large amounts of Na+ from the shoot and playing a crucial role in plant tolerance to salt.

A mutation leading to the total lack of nitrite reductase activity in Escherichia coli K 12

SummaryMutants of E. coli, completely devoid of nitrite reductase activity with glucose or formate as donor were studied. Biochemical analysis indicates that they are simultaneously affected in nitrate reductase, nitrite reductase, fumarate reductase and hydrogenase activities as well as in cytochrome c552 biosynthesis. The use of an antiserum specific for nitrate reductase shows that the nitrate reductase protein is probably missing. A single mutation is responsible for this phenotype: the gene affected, nir R, is located close to tyr R i.e. at 29 min on the chromosomal map.

A Mutant of Escherichia coli Deficient in Pyruvate Formate Lyase

SummaryA genetic study was performed on a mutation which has caused loss of pyruvate formate lyase. The gene affected is designated pfl and is located close to ser C, i.e. at 20 mn on the chromosomal map of E. coli.

Formate: A new electron donor for nitrite reduction in Escherichia coli K12

Abstract Reduction of nitrite by unbroken cells of Escherichia coli occurs with formate as electron donor. This activity constitutes about 25 % of the overall nitrite reduction. Electron transfer is performed through the formate dehydrogenase and, probably, a cytochrome-containing chain.

Genetic analysis of mutants of Escherichia coli K12 and Salmonella typhimurium LT2 deficient in hydrogenase activity

SummaryA genetic, study of mutants deficient in hydrogenase activity was performed. In E. coli, the affected gene (hyd) is located at 51 min, between cys C and nal B; in S. typhimurium, it probably lies in the homologous region of the chromosome.

Mapping of the gene Chl-B controlling membran bound nitrate reductase and formic hydrogen-lyase activities in Escherichia coli K 12

Abstract Chl r mutants lacking nitrate reductase A and formic hydrogene-lyase have been studied. Chl B maps near met E, its function is probably to code for a protein essential for the formation of “chlorate particle” in anaerobic conditions.

Morphological mutants of Escherichia coli K12

SummaryMorphological env C mutation affects the septum and leads to chain formation. Genetic analysis of a non-conditional env C mutant is performed; the gene order found is xyl-mtl-env C-pyr E.

Comparison between the chromosomal maps of Escherichia coli and Salmonella typhimurium lenght of the inverted segment in the trp region

SummaryGenetic analysis of nitrate and tetrathionate reductase-less mutants of Salmonella typhimurium and the comparison of the trp region of this organism with that of Escherichia coli lead to the conclusion that the inversion involves the whole segment between ttr and chl C genes, i.e. approximately 10 percent of the genome.

Genetic analysis of mutants from Escherichia coli K12 unable to grow anaerobically without exogenous acceptor

SummaryThe ana mutation leads E. coli to need an exogenous electron acceptor for anaerobic growth. The affected gene maps near 26 min between chlC and tdk on the chromosome of the organism.

A Mutant of Salmonella typhimurium Deficient in Tetrathionate Reductase Activity

SummaryA genetical study was made of a mutation leading to the specific loss of tetrathionate-reductase activity. The gene affected is called ttr and is located at 46 min on the chromosomal map of Salmonella typhimurium.