Control of pigmentation of Ustilago hordei:: The effect of pH, thiamine, and involvement of the cAMP cascade

The intensely pigmented teliospores of Ustilago hordei that are produced on susceptible barley cultivars contain nondiffusible deposits of a melanin-like pigment. Expression of pigmentation differed among haploid sporidial cultures and could be shown to be influenced by culture conditions. Pigmentation of strain 8.9a was black at acidic pH and repressed in medium adjusted to neutral or basic pH, and growth at elevated pH triggered a concomitant accumulation of a diffusible red pigment. Pigment formation by some strains was also determined to be under the control of thiamine and was completely inhibited when thiamine was present at levels above 0.02 mu M. The second messenger, cAMP transiently repressed pigment formation, whereas the cAMP phosphodiesterase inhibitor isobutryl-3-methyl xanthine completely repressed pigment formation. Like cAMP, the expression of the G alpha subunit gene FIL1 from a multicopy vector resulted in transient inhibition of pigment formation. However, pigment formation was not observed in cells expressing the mutant allele FIL1(Q206R), which ostensibly renders the gene constitutively active. The means by which pigment formation is repressed suggested that numerous genes were involved. Upon examination of a wild-type strain transformed with random cosmid clones of a genomic library, it was estimated that approximately 30 cosmid members per genome equivalent caused repression of the melanin-like pigments, whereas approximately 6 cosmid members induced pigment formation. (C) 1998 Academic Press.