Complementable mutants of could be ‘rescued’ to form heat-resistant spores by fusing them with sporulating cells of the wild-type strain (Dancer & Mandelstam, 1981). The latter strain acted as a ‘rescuer’. Rescuer protoplasts had to be derived from cells grown in a medium that allowed rapid and synchronous sporulation subsequently. Even protoplasts made from vegetative cells growing in such a medium ( protoplasts) acted as rescuers. Therefore, it was not necessary for the rescuer cells to have initiated sporulation before they were converted to protoplasts.

The genetic requirement for rescuer protoplasts was that they were for the gene to be complemented. Strains in the same complementation group did not rescue one another. Protoplasts of strains blocked at stages O, IV and V of sporulation acted equally well as rescuers whether they were converted to protoplasts at or (i.e. 3 h after transfer to sporulation medium). However, protoplasts of stage II and stage III mutants were much less effective rescuers than their counterparts. This difference was not due to a dominance effect expressed only in protoplasts.


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