24K Protein - A Genetic Marker for the Symbiotic Region of Rhizobium Leguminosarum

When grown on agar slants, all strains of Rhizobium leguminosarum so far examined synthesised large amounts of a polypeptide, molecular weight 24kdal, for which the structural gene is located on the symbiotic plasmid. In the symbiotic plasmid pRLlJI, it maps between determinants for nodulation and the structural genes for nitrogenase. Although characteristic of R. leguminosarum, the gene is inessential for symbiotic N2 fixation and has no known phenotype. The gene product, which can be detected by a simple antibody staining reaction (figure 1), provides a convenient genetic marker flanking the symbiotic region. Using this method, cosmids carrying cloned symbiotic genes have been isolated from gene banks and transfer of natural symbiotic plasmids between strains has been monitored. In addition, the structural gene for 24K protein has been used as a target for site- directed reversed mutagenesis with Tn5 to provide a selectable marker at a convenient place on natural symbiotic plasmids.