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. 1). In part, this conclusion is based on the failure to find free DNA copies of sequences from HML or HMR in yeast.

Fig1. Folding the chromosome to bring HMR near MAT so copied sequence information can be passed directly from HMR to MAT.

One simple method for expressing sequences at MAT and not expressing 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 explanation, however, for transcription of α and a genes begins from within the mating type-specific Y sequences. Another possibility is that an enhancer 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.

Since it is hundreds of nucleotides from the mating type-specific promoter to sequences unique to the HML, HMR or MAT loci, activation or repression would have to occur over a substantial distance (Fig. 2). 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.

Fig2. Something has to act at a distance to regulate the promoters of mating type genes. Either repression acts from afar at HML and HMR or activation acts over a distance at MAT.

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أوبن أيه آي تعتزم إطلاق متصفح إنترنت يعمل بالذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Sterile Mutants, Membrane Receptors and G Factors

Isolation of α2 Protein

Silencing HML and HMR

Receptor Tyrosine Kinases, Phosphoinosite-3-Kinase, and Mitogen-Activated Protein Kinase Pathways

Mating Type Conversion in Saccharomyces cerevisiae

The Yeast Cell Cycle

Structure and Function of TFIIIA

Switching from Oocyte to Somatic 5S Synthesis

Genome Editing of Immune Effector Cells for Safer and More Potent Cancer Therapies (CAR-T Cells)

Therapeutic Genome Editing for Hematologic Diseases: β-Thalassemia

Therapeutic Genome Editing for Hematologic Diseases: Sickle Cell Disease

TFIIIA Binding to the Middle of its Gene as Well as to RNA