No abstract

SUMMARY The formation of mitotically derived spores, called conidia, is a common reproductive mode in filamentous fungi, particularly among the large fungal class Ascomycetes. Asexual sporulation strategies are nearly as varied as fungal species; however, the formation of conidiophores, specialized multicellular reproductive structures, by the filamentous fungus Aspergillus nidulans has emerged as the leading model for understanding the mechanisms that control fungal sporulation. Initiation of A. nidulans conidipohore formation can occur either as a programmed event in the life cycle in response to intrinsic signals or to environmental stresses such as nutrient deprivation. In either case, a development-specific set of transcription factors is activated and these control the expression of each other as well as genes required for conidiophore morphogenesis. Recent progress has identified many of the earliest-acting genes needed for initiating conidiophore development and shown that there are at least two antagonistic signaling pathways that control this process. One pathway is modulated by a heterotrimeric G protein that when activated stimulates growth and represses both asexual and sexual sporulation as well as production of the toxic secondary metabolite, sterigmatocystin. The second pathway apparently requires an extracellular signal to induce sporulation-specific events and to direct the inactivation of the first pathway, removing developmental repression. A working model is presented in which the regulatory interactions between these two pathways during the fungal life cycle determine whether cells grow or develop.

Sterigmatocystin (ST)

flbA encodes an Aspergillus nidulans RGS (regulator of G protein signaling) domain protein that is required for control of mycelial proliferation and activation of asexual sporulation. We identified a dominant mutation in a second gene, fadA, that resulted in a very similar phenotype to flbA loss‐of‐function mutants. Analysis of fadA showed that it encodes the alpha‐subunit of a heterotrimeric G protein, and the dominant phenotype resulted from conversion of glycine 42 to arginine (fadA(G42R)). This mutation is predicted to result in a loss of intrinsic GTPase activity leading to constitutive signaling, indicating that activation of this pathway leads to proliferation and blocks sporulation. By contrast, a fadA deletion and a fadA dominant‐interfering mutation (fadA(G203R)) resulted in reduced growth without impairing sporulation. In fact, the fadA(G203R) mutant was a hyperactive asexual sporulator and produced elaborate sporulation structures, called conidiophores, under environmental conditions that blocked wild‐type sporulation. Both the fadA(G203R) and the fadA deletion mutations suppressed the flbA mutant phenotype as predicted if the primary role of FlbA in sporulation is in blocking activation of FadA signaling. Because overexpression of flbA could not suppress the fadA(G42R) mutant phenotype, we propose that FlbA's role in modulating the FadA proliferation signal is dependent upon the intrinsic GTPase activity of wild‐type FadA.

toxins has significant health and economic costs. We A.nidulans ST pathway has led to identification of a 60 kb gene cluster (the ST Cluster; stc) that includes 25 coThe filamentous fungus Aspergillus nidulans contains regulated genes, many of which have been shown to a cluster of 25 genes that encode enzymes required to function in ST biosynthesis (Brown et al., 1996). Transynthesize a toxic and carcinogenic secondary metabolscription of all of these genes is dependent upon the ite called sterigmatocystin (ST), a precursor of the activity of aflR, a pathway-specific regulatory gene found better known fungal toxin aflatoxin (AF). One ST within the ST cluster that encodes a zinc binuclear clusterCluster (stc) gene, aflR, functions as a pathway-specific type DNA binding protein (Woloshuk et al., 1994; Yu transcriptional regulator for activation of other genes et al., 1996a). aflR expression is regulated during the life in the ST pathway. However, the mechanisms concycle such that aflR mRNA begins to accumulate early in trolling activation of aflR and synthesis of ST and AF the stationary phase and activation of other genes required are not understood. Here we show that one important for ST biosynthesis quickly follows (Yu et al., 1996a). level for control of stc gene expression requires genesNumerous observations have supported the hypothesis that were first identified as early acting regulators of that microbial secondary metabolite production and sporulasexual sporulation. Specifically, we found that lossation are intimately associated. The best known example of-function mutations in flbA, which encodes a RGS is found in Streptomyces spp., in which there are common domain protein, or dominant activating mutations in elements that regulate sporulation and antibiotic producfadA, which encodes the α subunit of a heterotrimeric tion (for a review see Hopwood, 1988). Similarly, earlier G protein, block both ST production and asexual observations suggested that the ability to complete wildsporulation. Moreover, overexpression of flbA or type asexual sporulation could be a prerequisite for ST/AF dominant interfering fadA mutations cause precocious biosynthesis in the filamentous fungus genus Aspergillus stc gene expression and ST accumulation, as well as (Kale et al., 1994(Kale et al., , 1996. To study the genetic basis unscheduled sporulation. The requirement for flbA in of the relationship between asexual development and sporulation and ST production could be suppressed by secondary metabolism and the possible common elements loss-of-function fadA mutations. The ability of flbA to in regulation of these two processes, we decided to activate stc gene expression was dependent upon examine the effects of various early acting developmental another early acting developmental regulator, fluG, mutations on ST production in A.nidulans. We previously and AflR, the stc gene-specific transcription factor.proposed that there are two antagonistic signaling pathThese results are consistent with a model in which ways regulating A....