The new class II transposon Tn163 is plasmid-borne in two unrelated Rhizobium leguminosarum biovar viciae strains

Ulrich A, Pühler A. The new class II transposon Tn163 is plasmid-borne in two unrelated Rhizobium leguminosarum biovar viciae strains. Molecular & General Genetics. 1994;242(5):505-516.Tn163 is a transposable element identified in Rhizobium leguminosarum bv. viciae by its high insertion rate into positive selection vectors. The 4.6 kb element was found in only one further R. leguminosarum bv. viciae strain out of 70 strains investigated. Both unrelated R. leguminosarum bv. viciae strains contained one copy of the transposable element, which was localized in plasmids native to these strains. DNA sequence analysis revealed three large open reading frames (ORFs) and 38 bp terminal inverted repeats. ORF1 encodes a putative protein of 990 amino acids displaying strong homologies to transposases of class II transposons. ORF2, transcribed in the opposite direction, codes for a protein of 213 amino acids which is highly homologous to DNA invertases and resolvases of class II transposons. Homology of ORF1 and ORF2 and the genetic structure of the element indicate that Tn163 can be classified as a class II transposon. It is the first example of a native transposon in the genus Rhizobium. ORF3, which was found not to be involved in the transposition process, encodes a putative protein (256 amino acids) of unknown function. During transposition Tn163 produced direct repeats of 5 bp, which is typical for transposons of the Tn3 family. However, one out of the ten insertion sites sequenced showed a 6 bp duplication of the target DNA; all duplicated sequences were A/T rich. Insertion of Tn163 into the sacB gene revealed two hot spots. Chromosomes of different R. leguminosarum bv. viciae strains were found to be highly refractory to the insertion of Tn163

An Fnr-like protein encoded in Rhizobium leguminosarum biovar viciae shows structural and functional homology to Rhizobium meliloti FixK.

Colonna-Romano S, Arnold W, Schlüter A, Boistard P, Pühler A, Priefer UB. An Fnr-like protein encoded in Rhizobium leguminosarum biovar viciae shows structural and functional homology to Rhizobium meliloti FixK. Mol Gen Genet. 1990;223(1):138-147.A 1.9 kb DNA region of Rhizobium leguminosarum biovar viciae strain VF39 capable of promoting microaerobic and symbiotic induction of the Rhizobium meliloti fixN gene was identified by heterologous complementation. Sequence analysis of this DNA region revealed the presence of two complete open reading frames, orf240 and orf114. The deduced amino acid sequence of orf240 showed significant homology to Escherichia coli Fnr and R. meliloti FixK. The major difference between ORF240 and FixK is the presence of 21 N-terminal amino acids in ORF240 that have no counterpart in FixK. A similar protein domain is also present in E. coli Fnr and is essential for the oxygen-regulated activity of this protein. Analysis of the nucleotide sequence upstream of orf240 revealed a motif similar to the NtrA-dependent promoter consensus sequence, as well as two DNA regions resembling the Fnr consensus binding sequence. A Tn5-generated mutant in orf240 lost the ability to induce the R. meliloti fixN-lacZ fusion. Interestingly, this mutant was still capable of nitrogen fixation but showed reduced nitrogenase activity

FUNCTIONAL-ANALYSIS OF THE CYSTEINE MOTIFS IN THE FERREDOXIN-LIKE PROTEIN FDXN OF RHIZOBIUM-MELILOTI INVOLVED IN SYMBIOTIC NITROGEN-FIXATION

MASEPOHL B, KUTSCHE M, RIEDEL KU, SCHMEHL M, KLIPP W, Pühler A. FUNCTIONAL-ANALYSIS OF THE CYSTEINE MOTIFS IN THE FERREDOXIN-LIKE PROTEIN FDXN OF RHIZOBIUM-MELILOTI INVOLVED IN SYMBIOTIC NITROGEN-FIXATION. MOLECULAR & GENERAL GENETICS. 1992;233(1-2):33-41.The Rhizobium meliloti fdxN gene, which is part of the nifA-nifB-fdxN operon, is absolutely required for symbiotic nitrogen fixation. The deduced sequence of the FdxN protein is characterized by two cysteine motifs typical of bacterial-type ferredoxins. The Fix- phenotype of an R. meliloti fdxN::[Tc] mutant could be rescued by the R. leguminosarum fdxN gene, whereas no complementation was observed with nif-associated genes encoding ferredoxins from Bradyrhizobium japonicum, Azotobacter vinelandii, A. chroococcum and Rhodobacter capsulatus. In addition to these heterologous genes, several R. meliloti fdxN mutant genes constructed by site-directed mutagenesis were analyzed. Not only a cysteine residue within the second cysteine motif (position 42), which is known to coordinate the Fe-S cluster in homologous proteins, but also a cysteine located downstream of this motif (position 61), was found to be essential for the activity of the R. meliloti FdxN protein. Changing the amino acid residue proline in position 56 into methionine resulted in a FdxN mutant protein with decreased activity, whereas changes in positions 35 (Asp35Glu) and 45 (Gly45Glu) had no significant effect on the function of the FdxN mutant proteins. In contrast to bacterial-type ferredoxins, which contain two identical cysteine motifs of the form C-X2-C-X2-C-X3-C, nif-associated ferredoxins, including R. meliloti FdxN, are characterized by two different cysteine motifs. Six "additional" amino acids separate the second (Cys42) and the third cysteine (Cys51) in the C-terminal motif (C-X2-C-X8-C-X3-C). By molecular modelling, it was predicted that these amino acid residues form a loop, which does not alter the relative positions of the neighbouring cysteines. Deletion of this loop resulted in an R. meliloti FdxN mutant protein, which exhibited almost 70% wild-type activity, indicating that the predicted loop does not affect Fe-S cluster binding and plays no crucial role in activity of the FdxN protein

A DEFINED AMINO-ACID EXCHANGE CLOSE TO THE PUTATIVE NUCLEOTIDE BINDING-SITE IS RESPONSIBLE FOR AN OXYGEN-TOLERANT VARIANT OF THE RHIZOBIUM-MELILOTI NIFA PROTEIN

KREY R, Pühler A, KLIPP W. A DEFINED AMINO-ACID EXCHANGE CLOSE TO THE PUTATIVE NUCLEOTIDE BINDING-SITE IS RESPONSIBLE FOR AN OXYGEN-TOLERANT VARIANT OF THE RHIZOBIUM-MELILOTI NIFA PROTEIN. MOLECULAR & GENERAL GENETICS. 1992;234(3):433-441.In Rhizobium meliloti the NifA protein plays a central role in the expression of genes involved in nitrogen fixation. The R. meliloti NifA protein has been found to be oxygen sensitive and therefore acts as a transcriptional activator only under microaerobic conditions. In order to generate oxygen-tolerant variants of the NifA protein a plasmid carrying the R. meliloti nifA gene was mutagenized in vitro with hydroxylamine. About 70 mutated nifA genes were isolated which mediated up to 12-fold increased NifA activity at high oxygen concentrations. A cloning procedure involving the combination of DNA fragments from mutated and wild-type nifA genes allowed mapping of the mutation sites within the central part of the nifA gene. For 17 mutated nifA genes the exact mutation sites were determined by DNA sequence analysis. It was found that all 17 mutated nifA genes carried identical guanosine adenosine mutations resulting in a methionine - isoleucine exchange (M217I) near the putative nucleotide binding site within the central domain. Secondary structure predictions indicated that the conformation of the putative nucleotide binding site may be altered in the oxygen-tolerant NifA proteins. A model is proposed which assumes that at high oxygen concentrations the loss of activity of the R. meliloti NifA protein is due to a conformational change in the nucleotide binding site that may abolish binding or hydrolysis of the nucleotide. Such a conformational change may be blocked in the oxygen-tolerant NifA protein, thus allowing interaction with the nucleotide at high oxygen concentrations

The Rhizobium meliloti fdxN gene encoding a ferredoxin-like protein is necessary for nitrogen fixation and is cotranscribed with nifA and nifB

Klipp W, Reilander H, Schlüter A, Krey R, Pühler A. The Rhizobium meliloti fdxN gene encoding a ferredoxin-like protein is necessary for nitrogen fixation and is cotranscribed with nifA and nifB. MOLECULAR & GENERAL GENETICS. 1989;216(2-3):293-302