Cells of opposite mating type, a1 and a2, of the anther smut, Ustilago violacea assemble a conjugation tube after about 3-4 h of mating on nutrient-free media. Low doses of ultraviolet light (u.v.) delay but do not prevent conjugation in wild-type strains if given in the first 2-3 h of mating. However, irradiation later than this period has little effect and conjugation proceeds normally. The u.v. effect is photoreactivable and we conclude that u.v. induces dimers which affect transcription of specific messenger RNA molecules needed for conjugation (‘sex message’). Our evidence suggests that the dimers may cause mistranscription rather than the complete prevention of transcription. The effect of u.v. on conjugation in reciprocal crosses of u.v.-sensitive and wild-type strains indicates clearly that both partners must complete transcription of sex messages in order to conjugate. Inactivation of either partner before transcription prevents conjugation, but conjugation proceeds when either cell is inactivated after transcription of the sex messages has occurred. These results suggest that a mutual and reciprocal exchange of information between the 2 mating-types occurs prior to the assembly of the conjugation tube.

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