Segregation pattern of kanamycin resistance marker in Azotobacter vinelandii did not show the constraints expected in a polyploid bacterium.

It was suggested that Azotobacter vinelandii cells contain about 80 copies of their chromosome and when foreign genes are introduced into the cell, it took several generations for them to spread to all 80 chromosomes even in the presence of selection. In contrast, the fact that many recessive mutants can be isolated from A. vinelandii without the constraints expected for a cell that has 80 copies of its chromosome argued against this organism being highly polyploid. We have investigated the segregation of a kanamycin resistant genetic marker under non-selective conditions in A. vinelandii. Plasmid DNA was used to introduce the kanamycin resistance gene onto the A. vinelandii chromosome at the nifY locus by homologous recombination. The transformants were identified from non-transformants with the aid of replica plating, and hence the colonies examined for segregation of the genetic marker were never subjected to kanamycin selection. In spite of growing the transformants in the absence of selection pressure, no segregant that lacked the kanamycin resistance gene was scored. These analyses suggested that the segregation of the kanamycin marker in A. vinelandii did not exhibit any constraints expected in a highly polyploid bacterium.