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106 CHAPTER 5

The MATlocus is a regulatory locus, governing the

expression of several genes. These include:

• The genes responsible for producing hormones
  termed a-factorand αα-factor, consisting of 12 and
  13 amino acids respectively:
  
  
  • a-Factor: NH 2 -Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-
    Gly-Gln-Pro-Met-Tyr-COOH;
  
  
  
  


• αα-Factor: NH 2 -Tyr-Ile-Ile-Lys-Gly-Val-Phe-Trp-Asp-
  Pro-Ala-Cys(S-farnesyl)-COOCH 3.


• The genes that code for hormone receptors on the
  cell surface.


• The genes that code for cell surface agglutinins.

The ααstrains produce αα-factor constitutively and it
diffuses to acells where it is recognized by a specific
receptor. This receptor binding causes the growth of a
cells to be arrested at “start” during G1 of the cell cycle

• the only stage at which they are competent to mate
  (Chapter 4). The astrain then produces a-factor which
  diffuses to the ααstrain where it is, again, recognized
  by a specific receptor. Receptor binding in both cases
  leads to other changes in the cells: they produce short
  outgrowths which function as conjugation tubes, and
  the surfaces of these are covered by a strain-specific
  glycoprotein, so that when an acell and an ααcell make
  contact they adhere tightly by their complementary
  agglutinins. The conjugation tubes then fuse, and the
  two nuclei fuse to form a diploid cell. At this stage the
  cell can go on to produce a diploid budding colony
  or it can undergo meiosis, depending on whether
  the environmental conditions are suitable for growth.
  An essentially similar hormone and receptor system
  to this is found in other yeasts such as Pichiaand
  Hansenulaspp, and in the fission yeast Schizosaccharo-
  myces pombe.
  The sex pheromones of several mycelial Ascomycota
  and Basidiomycota have now been characterized.
  They are peptides, such as the αα-factor of S. cerevisiae,
  or more often lipopeptidessuch as the a-factor of S.
  cerevisiae. There is considerable variation in the amino
  acid substitutions of these pheromones, but their basic
  structures are similar. For example, the jelly fungus
  Tremella mesenterica(Basidiomycota; see Fig. 2.7) pro-
  duces a pheromone termed tremerogen. This is a
  highly lipophilic molecule with the structure shown
  below. It closely resembles the a-factor of S. cerevisiae:

Tremerogen A-10: NH 2 -Glu-His-Asp-Pro-Ser-Ala-Pro-
Gly-Asn-Gly-Tyr-Cys(S-farnesyl)-COOCH 3

The tetrapolar compatibility system in Basidiomycota

Most Basidiomycota are heterothallic, with mating-

type genes at either one locus or two loci, termed A

and B. There are multiple idiomorphs at each locus,

and a successful mating will occur between any two

strains that differ from one another at each locus. This

greatly increases the chances of finding a mate, com-

pared with fungi that have only two idiomorphs.

The basic mating behavior of Basidiomycota was

outlined in Chapter 2 (see Fig. 2.18). Strains derived

from haploid basidiospores are monokaryons and they

can fuse with other compatible monokaryons to form

a dikaryon. All subsequent growth involves the syn-

chronous division of the two nuclei in each hyphal

compartment and their regular distribution as nuclear

pairs throughout the mycelium. In several members

of the group this regular arrangement is aided by the

production of clamp connections (see Fig. 2.20). Even-

tually, the dikaryotic colony will produce a fruitbody,

and nuclear fusion and meiosis occur in the basidia.

By experimentally pairing strains with the same A or

the same B ideomorph, it has been possible to deduce

the regulatory roles of the A and B loci (Casselton et al.

1995). As summarized in Table 5.2, pairings of fully

compatible strains (with different A and different B) lead

to reciprocal exchange of nuclei, because the dolipore

septa which normally prevent nuclear migration are

digested, and clamp branches are formed at each sep-

tum to fuse with the compartment behind. In pairings

of strains with the same A and same B (common A,

common B) there is no septal breakdown, no nuclear

migration, no dikaryotization, and no clamp connec-

tions. Pairings of strains with different A but common

B loci show nuclear pairing, synchronous division of the

nuclei and formation of clamp branches; but the dolipore

septa do not break down, and the clamp branches do

not fuse with the parent hypha. Pairings of strains with

different B but common A lead to septal dissolution

but none of the other events. Thus, the A locus(com-

Table 5.2Roles of mating-type genes in basidiomycota.

Pairing of strains with Events observed

Different A, different B 1 Septal dissolution

idiomorphs (dikaryon) 2 Nuclear migration

3 Clamp branches arise

and fuse with hypha

Common A, different B 1 Septa dissolve

idiomorphs 2 Nuclei migrate

Common B, different A 1 Septa remain intact

idiomorphs 2 No nuclear migration

3 Clamp branches arise

but do not fuse

Common A, common B 1 No septal dissolution

idiomorphs 2 No nuclear migration

3 No clamp connections