Genetic analysis of the double resistance phenotype to β(1,3)D-glucan synthase from the Schizosaccharomyces pombe mutant pbr1-8

The cell wall can be considered a specific target in the development of new antifungal agents. The biosynthesis of (1,3)β-D-glucan, one of the structural elements of the fungal cell wall, represents a valuable model for morfogenetic studies. We have identified four bgs (beta glucan synthesis) genes that probably are the catalytic subunits of four different (1,3)β-D-glucan synthase activities in S. pombe. bgs4⁺ was cloned from a cosmid clone located in the same region where we described two genetically linked mutant genes, both related with the
(1,3)β-D-glucan synthesis: cwg1-1, a termosensitive mutant affected in the (1,3)β-D-glucan synthase catalytic subunit and pbr1-8, implicated in the resistance to antifungal drugs that specifically inhibit the (1,3)β-D-glucan synthase activity.
We found that cwg1⁺ and bgs4⁺ are the same gene. On the other hand, bgs4⁺ does not suppress the recessive phenotype of a pbr1–8 mutant. By the contrary, a bgs4D/pbr1–8 diploid, containing a plasmid expressing bgs4⁺ under the control of the thiamine-repressible nmt1⁺ promoter 81X, is resistant to the antifungals when the promoter is repressed and sensitive when it is induced. Besides, bgs4⁺ cloned from a pbr1–8 mutant, induces a resistant phenotype in a bgs4D but not in a bgs4⁺ strain. Therefore, bgs4⁺ is at least partially responsible for the resistant phenotype of pbr1–8 mutant. These results suggest that it is possible the existence of a second gene, very tightly linked to bgs4⁺, that would be responsible for an additional resistance phenotype. We performed a bgs4⁺ deletion analysis from a pbr1–8 strain containing the thiamine-repressible plasmid p81X-bgs4⁺, to maintain the deletion strain viable. Deletion of the complete bgs4^pbr1-8 ORF suppressed the mutant phenotype. However, deletions of (1) 1 Kb promoter region plus ATG codon, (2) 200 bases of 5
end ORF, or (3) 1 Kb of 3
end ORF showed, in the three cases, that the truncated Bgs4p were not functional but the resulting bgs4^pbr1-8 strains displayed the resistance phenotype. We are currently performing new bgs4⁺ partial deletions and complementation tests. These and new results will be discussed.