Expression and Recombination Paradoxes

The Expression and Recombination Paradoxes

Two important questions are raised by the structure of the mating-type loci and the locations of transcripts in MATa and MATα. First, how is transposition always forced to proceed from HML or HMR to MAT and never the reverse, and second, how is expression of a or α from HML or HMR prevented?

Relatively little is known about the biochemistry of the transposition reaction. The available data suggest that a direct transfer of DNA through a free diffusible intermediate does not occur, but instead that mating-type shift is a result of gene conversion  type of substitution reaction in which the appropriate information from HML or HMR is copied into MAT (Fig. 16.6). In part, this conclusion is based on the failure to find free DNA copies of sequences from HML or HMR in yeast.

Figure 16.6 Folding the chromosome to bringHMRnearMATso copiedsequence information can be passed directly from HMR to MAT.

One simple method for expressing sequences at MAT and not express-ing the sequences at HML or HMR would have been for the region around MAT to provide a promoter that specifies transcription across whatever sequence has been inserted at MAT. This is not the explana-tion, however, for transcription of α and a genes begins from within the mating type-specific Y sequences. Another possibility is that an en-hancer is near the MAT region so that whatever is inserted into MAT is expressed. It is also possible that repression occurs at HML and HMR.

Figure 16.7 Something has to act at a distance to regulate the promoters ofmating type genes. Either repression acts from afar at HML and HMR or activation acts over a distance at MAT.

Since it is hundreds of nucleotides from the mating type-specific pro-moter to sequences unique to the HML, HMR or MAT loci, activation or repression would have to occur over a substantial distance (Fig. 16.7). While such “action at a distance” events are frequently found for activation of transcription, they are less often seen for repression of transcription. Nonetheless, genetics experiments revealed that it is repression at HML and HMR that actually occurs. This same repression or silencing mechanism of the mating-type promoters also blocks trans-fer of the donor information at HML and HMR to MAT.