Dictyostelium discoideum
There are two strains of "Pseudomonas fluorescens" associated with "Dictyostelium discoideum." One strain serves as a food source and the other strain does not. The main genetic difference between these two strains is a mutation of the global activator gene called gacA. This gene plays a key role in gene regulation; when this gene is mutated in the nonfood bacterial strain, it is transformed into a food bacterial strain.