The mating type locus chr iii
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The mating type locus Chr. III. The MAT locus information. The MAT locus can encode three regulatory peptides: - a 1 is encoded by the MAT a allele -  1 and  2 are encoded by the MAT  allele Three regulatory activities:  1,  2, and a 1-  2.

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The mating type locus chr iii

The matingtype locusChr. III


The mat locus information
The MAT locus information

  • The MAT locus can encode three regulatory peptides:

  • - a1 is encoded by the MATa allele

  • - 1 and 2 are encoded by the MAT allele

  • Three regulatory activities: 1, 2, and a1-2.


Sterile mutants can monitor the mat status
Sterile mutants can monitor the MAT status

  • Mutations have been identified at several loci that produce a non-mating phenotype, called sterile (STE).

  • The sterile mutations fall into three classes:

    • 1. sterility only in a cells

      • - STE2, the a pheromone receptor

    • 2. sterility only in  cells

      • - STE3, the  pheromone receptor

    • 3. sterility in both a and  cells

      • - STE12, the general pheromone-responsive transcription factor


Saccharomyces as a model system
Saccharomyces as a model system

  • How do cells generate a mitotically stable, complex, specific cell type?

  •  same DNA, but different gene expression states.

  • How do cells respond to environmental change or information from other cells?

    • decision-making algorithms.

      • How do cells maintain an undifferentiated state “stem cell”?

  •  non-equivalence of daughter cells at mitosis.


  • Sterile mutants can monitor the mat status1
    Sterile mutants can monitor the MAT status

    • The STE genes can be used to track the effects of mutations at other loci, such as MAT.

    • STE response be measured as fertility/sterility (mating).

    • Or, reporter gene constructs made with the transcriptional response elements from STE genes can drive the E. coli-galactosidase gene.

    • The reporter gene is visualized on screens by the ability to metabolize XGAL to a blue color, giving blue (gene active) or white (gene inactive) colonies.

    STE12 response

    b-galactosidase


    Mat regulation in cells
    MAT regulation in  cells

    • When the  allele is present at MAT, two genes are expressed: MAT1 and MAT2,

    • Mutations in 1 affect only -specific genes, such as STE3.

    • MAT1 mutants prevent normal expression of STE3.

    • They do not affect other haploid specific genes or a-specific genes.

    •  1 is a positive regulator of -specific genes

    • Mutations in 2 allow the expression of a-specific genes, even in a MAT cell.

      • 2 is a negative regulator of a-specific genes

    • Consequently, in a MAT cell the  genes are expressed while the a genes are not.



    The yeast genome
    The yeastgenome

    • First eukaryotic genome sequenced, April 1996

    • Consortium effort, US / EU

    • 16 well characterized chromosomes

    • PFGE separation of chromosomes

    • Chr. I (230 kb) <-> chr. IV (1532 kb)

    • 13 Mb (3.5 x coli)

    • 6183 ORFs > 99 aa

    • 72% coding ! (<2% human)

    • Average ORF 1450 bp

    • Few introns (<4% of ORFs)

    • 1/3 of ORFs characterized

    • 1/3 of ORFs have homologies, motifs

    • 1/3 of ORFs have unknown function

    • 120 rRNA copies of 9137 bp on chr. XII

    • 262 tRNAs


    Essential genes
    Essential genes

    • About 1000 of the 6100 ORF are essential genes

    • Test for essential gene:

      • Gene disruption in diploid

      • Sporulation and tetrad dissection

      • 2 viable 2 dead spores

    • Many genes would be essential in nature that are dispensable on laboratory rich media

      • eg. carbon source

      • Temperature

      • Salts...


    The genetic and physical map of chromosome iii
    The genetic and physical map of chromosome III

     Both strands contain about the same number of ORFs

     Often several ORFs on one strand not interrupted by ORFs on the other strand

     Very few overlapping ORFs on the same strand

     No overlap of divergently transcribed ORFs

     Close shared promoters of divergently transcribed ORFs

     Most DNA is ORF

     Few and small introns

     Genes close to the centromer


    The mating type locus chr iii

    chromosome III (cont.)

     Dispersed tRNAs (270 / genome)

     Ty elements and there remnants are 5’ to tRNAs

     Dispersed snRNAs and snoRNAs

     3kb / cM (200x less than in humans)

     rRNA on Chr XII, no recombination, nucleolus remains associated with the chromosome during meiosis

     Moderate suppression of recombination around centromeres

     Genome 4300 cM -> 45 x 2 crossovers per meiosis


    Genetic nomenclature
    Genetic nomenclature

    • 3 letter name with digit, eg. CDC33, CMD1

    • Italic = genes

    • Uppercase = wild-type

    • Loss of function, cdc33

    • cdc33-1, known allele

    • Cdc33 protein, Cdc33p

    • Phenotypes TS+, ts-

    • arg2∆,arg2::LEU2

    • Dominant / recessive

    • ORF designation YCR49w


    The mating type locus chr iii

    The genome duplication

    • whole genome duplication 100 mio years ago (polyploidy; Ohno’s hypothesis, shark)

       Tetraploid -> diploid + many deletions and reciprocal translocations

    • 55 duplicate regions, 13% of all ORFs, 50% of the genome !

    • functions in anaerobiosis ?