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Candida albicans is a polymorphic human opportunistic pathogen in which the Swi–Snf complex functions as an activator whereas Tup1 acts as a general repressor during the yeast-hyphae transition. In Saccharomyces cerevisiae, the interplay between the Swi–Snf complex and the Tup1–Ssn6 repressive complex regulates the balance between active and repressed chromatin structures of a number of genes. To study the interplay between Candida albicans Swi1 and Tup1 and their effects on morphogenesis, we analyzed phenotypes of swi1/ swi1, tup1/ tup1 and swi1/ swi1 tup1/ tup1 mutants under various growth conditions. The swi1/ swi1 mutant failed to form true hyphae, whereas the tup1/ tup1 mutant exhibited constitutive filamentous growth. Deletion of SWI1 in the tup1/ tup1 mutant completely blocked hyphal growth under all the conditions examined. Under aerobic conditions, the swi1/ swi1 tup1/ tup1 mutant most resembled the swi1/ swi1 mutant in phenotype, actin polarization and gene expression pattern. In invaded agar, the double mutant showed similar phenotypes as the swi1/ swi1 mutant, while under embedded conditions, it grew as a pseudohypha-like form different from that of the wild-type strain, swi1/ swi1 or tup1/ tup1 mutants. These results suggest that Swi1 may play a dominant role by antagonizing the repressive effect of the Tup1 on hyphal development in C. albicans. [ABSTRACT FROM AUTHOR]