Conservation of nif sequences in Frankia

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Nitrogen fixation (nif) genes are crucial for symbiotic relationships between Frankia bacteria and actinorhizal plants.
• Understanding the genetic diversity of Frankia nif genes is essential for improving nitrogen fixation efficiency in agriculture and forestry.

Purpose of the Study:

• To investigate the genetic diversity of nitrogen fixation (nif) genes in various Frankia strains.
• To compare the sequence conservation and organization of nifHDK genes across different Frankia strains and host compatibilities.

Main Methods:

• Southern blot hybridization using nifHDK probes from Rhizobium meliloti, Klebsiella pneumoniae, and Frankia strain Arl3.
• Construction and screening of gene banks for Frankia strains (EUN1f, HRN18a, CeD, ACoN24d) to isolate nif-hybridizing fragments.
• Nucleotide sequencing of the nifH open reading frame (ORF) from Frankia strain Arl3.

Main Results:

• Observed variations in restriction fragment sizes among different Frankia strains hybridized with nifHDK probes.
• Greater sequence divergence noted in nif genes of Elaeagnus-compatible Frankia strains compared to Alnus- or Casuarina-compatible strains.
• Identified the nifH zone as highly conserved, with nifH and nifD genes found to be contiguous. The nifH ORF from Frankia strain Arl3 is 861 bp, encoding a 287-amino acid polypeptide.

Conclusions:

• Frankia nifH gene sequences exhibit significant strain-specific variations, correlating with host plant compatibility.
• Frankia shares closer evolutionary relationships with cyanobacteria (Anabaena) and other Gram-negative bacteria (Azotobacter, K. pneumoniae) based on nifH sequence comparisons.
• Frankia shows the least sequence similarity in nifH genes to Gram-positive bacteria (Clostridium pasteurianum) and archaea (Methanococcus voltae).