In bacteriophage T4, all rIIA mutants are found to complement all rIIB mutants. However, rIIA mutants fail to complement other rIIA mutants, and rIIB mutants fail to complement other rIIB mutants
Furthermore, mutants with deletions that span both rIIA and rIIB complementation groups do not complement either A or B mutants. What do these data mean, and what do the complementation groups represent?
Complementation occurs in crosses between mutants of the different complementation groups, which represent functional units, or genes. That is, rIIA mutants produce functional B gene products, and rIIB mutants produce functional A gene products. Both A and B gene products are required to produce a wild-type phenotype. Mutants with deletions that span both complementation regions cannot recover the wild-type phenotype because they will always be missing part of a functional unit.
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