We have determined the nucleotide sequences of two genes from Klebsiella pneumoniae, nifA, the nif‐specific activator of transcription and ntrC, the bifunctional regulatory protein involved in ‘nitrogen control’. These sequences differ significantly from those previously published. In particular, nifA extends 40 codons beyond the stop codon reported earlier. This extension encodes a putative DNA‐binding domain strongly homologous to the Rhizobium meliloti nifA protein and to some extent to the ntrC protein. In all three proteins this domain is linked by a segment of variable length to a strongly conserved central domain of 240 residues. A short segment having the properties of an interdomain linker joins the central region to an N‐terminal domain, which is weakly related in the case of the two nifA proteins. This homology is not shared by the N‐terminal domain of ntrC, which is clearly but unexpectedly related to the N‐terminal domains of a diverse set of procaryotic pleiotropic control proteins, including ompR, dye and nusA from Escherichia coli and spoOA and spoOF from Bacillus subtilis.

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Leaves from over 1000 Brazilian native plants growing in the cerrado and neighbouring regions were sampled for C and N content. Half of these were analysed for N and further samples forC and ash content. Nodulated legumes from all three sub-families were included, together with two types of reference plant, non-nodulated legumes and non-legumes. Particular emphasis was placed on the large caesalpinioid genus Chamaecrista which is here for the first time reported to fix nitrogen in its native habitats. Woody and herbaceous species of this and other nodulated genera, with the exception of the mimosoid tree Stryphnodendron, showed evidence of nitrogen fixation. Amounts fixed were site-specific as was the N signature of reference plants. There was no evidence that nodulated legumes had higher leaf N than non-nodulated legumes: both were higher than non-legumes. Several species of Chamaecrista from section absus and species of Stryphnodendron had carbon contents of 50-55%, higher than previously reported for leaves. This was coupled with low (1-3%) ash contents. TheC values of plants with ≥49% C were significantly more negative than those with <49% C: most species in the former group were woody and most in the latter group herbaceous. Mimosa pudica was unusual in having a wide range of percent C, percent ash and C values; these parameters were significantly correlated. It is concluded that Brazilian native legumes can fix significant amounts of nitrogen in the nutrient-poor cerrado soils. Consideration of mineral and lipid nutrition will be necessary in order fully to understand relations betweenC, carbon content and other physiological parameters.

The mob mutants in Escherichia coli are pleiotropically defective in all molybdoenzyme activities. They synthesise molybdopterin, the unique core of the molybdenum cofactor, but are unable to attach the GMP moiety to molybdopterin to form molybdopterin guanine dinucleotide, the functional molybdenum cofactor in Escherichia coli. A partially purified preparation termed protein FA (protein factor d'association), is able to restore molybdoenzyme activities to broken cell preparations of mob mutants. A fragment of DNA capable of complementing mob mutants has been isolated from an E. coli genomic library. Strains carrying this DNA in a multicopy plasmid, express 30-fold more protein FA activity than the wild-type bacterium. Protein FA has been purified to homogeneity by a combination of ion-exchange, affinity and gel-filtration chromatography. Protein FA consists of a single polypeptide of molecular mass 22 kDa and is monomeric in solution. N-terminal amino acid sequencing confirmed that protein FA is a product of the first gene at the mob locus. The purified protein FA was required in stoichiometric rather than catalytic amounts in the process that leads to the activation of the precursor of the molybdoenzyme nitrate reductase, which is consistent with the requirement of a further component in the activation.Molybdoenzymes, with the exception of molybdodinitrogenase, contain a molybdenum cofactor which is composed in its simplest form of a unique pterin derivative, molybdopterin, to which the molybdenum is bound [l]. The chlorateresistant mutants of Escherichia coli (recently renamed mo [2]) are pleiotropically defective in all molybdoenzyme activities and are defective in the biosynthesis of active molybdenum cofactor. These mutants carry mutations which map to five distinct regions of the E. coli chromosome: moa, mob, mod, moe and mog. Each of the loci has been cloned and almost all have been sequenced [4-81. The overall functions encoded by most of the loci have been deduced. The moa and moe loci encode proteins which assemble the molybdopterin portion of the cofactor [9, 101. The mod locus encodes the molybdate uptake system of the cell [9, 111. The function of mog is not known [12]. For several years the role of the mob locus in cofactor biosynthesis was unclear since these mutants possessed molybdopterin [13, 141. However, recently it has been demonstrated that in E. coli the active form of the cofactor is molybdopterin guanine dinucleotide (MGD), a covalent complex of molybdopterin with guanosine monophosphate. Mutants defective at mob are unable to attach GMP to molybdopterin, suggesting that the products of the mob locus are responsible for this late step in molybdenum cofactor biosynthesis [15].