SREB, a GATA Transcription Factor That Directs Disparate Fates in Blastomyces dermatitidis Including Morphogenesis and Siderophore Biosynthesis

Blastomyces dermatitidis belongs to a group of human pathogenic fungi that exhibit thermal dimorphism. At 22°C, these fungi grow as mold that produce conidia or infectious particles, whereas at 37°C they convert to budding yeast. The ability to switch between these forms is essential for virulence in mammals and may enable these organisms to survive in the soil. To identify genes that regulate this phase transition, we used Agrobacterium tumefaciens to mutagenize B. dermatitidis conidia and screened transformants for defects in morphogenesis. We found that the GATA transcription factor SREB governs multiple fates in B. dermatitidis: phase transition from yeast to mold, cell growth at 22°C, and biosynthesis of siderophores under iron-replete conditions. Insertional and null mutants fail to convert to mold, do not accumulate significant biomass at 22°C, and are unable to suppress siderophore biosynthesis under iron-replete conditions. The defect in morphogenesis in the SREB mutant was independent of exogenous iron concentration, suggesting that SREB promotes the phase transition by altering the expression of genes that are unrelated to siderophore biosynthesis. Using bioinformatic and gene expression analyses, we identified candidate genes with upstream GATA sites whose expression is altered in the null mutant that may be direct or indirect targets of SREB and promote the phase transition. We conclude that SREB functions as a transcription factor that promotes morphogenesis and regulates siderophore biosynthesis. To our knowledge, this is the first gene identified that promotes the conversion from yeast to mold in the dimorphic fungi, and may shed light on environmental persistence of these pathogens

NO/cGMP Signaling and HSP90 Activity Represses Metamorphosis in the Sea Urchin Lytechinus pictus

Volume: 201Start Page: 394End Page: 40

Tom D. Humphreys II: a pioneer of molecular embryology

Tom D. Humphreys, II began his career when we knew almost nothing about cellular recognition, gene expression, or patterning of embryos. He helped lead the research that rapidly progressed to the amazing state of knowledge we have today

Developmental Patterns of Cytoplasmic Transcript Prevalence in Sea Urchin Embryos

Several hundred two-cell stage, gastrula stage, and pluteus stage sea urchin embryo cDNA clones were screened with [^(32)P]cDNA transcribed from a developmental series of cytoplasmic and polysomal poly(A) RNAs. Transcript prevalence was estimated by reference to the reaction of a series of standard clones complementary to RNAs of known abundance. We describe the dominant pattern of prevalence change during development for abundant sequences, i.e., those present in the range 5 × 10^4–10^6 molecules/embryo. These transcripts are most often about equally abundant in egg and pluteus stage embryos, and are severalfold less prevalent at gastrula stage. Less than 10% of the sequences displaying this pattern undergo >10-fold changes in prevalence during development. A search for later embryo sequences not represented detectably in egg or cleavage stage embryo poly(A) RNA yielded a number of examples. These were confirmed by the RNA gel blot method. Sequences regulated in this manner all belonged to lower abundance classes, and are present at about 40 copies/cell or less at the gastrula stage

Cloned Embryo mRNAs Not Detectably Expressed in Adult Sea Urchin Coelomocytes

A set of 455 cloned pluteus cDNAs was analyzed by colony hybridization to radioactive cDNAs transcribed from cytoplasmic poly(A) RNA of pluteus embryos and adult coelomocytes. The prevalence of transcripts corresponding to each of the clones was estimated by use of cloned standards having known transcript prevalences. While 46% of the clones correspond to transcripts present at more than 105 copies/ng poly(A) RNA (about 5 copies/cell) in plutei, only 5.5% are that prevalent in the coelomocyte. The most prevalent pluteus transcripts are frequently also prevalent in coelomocytes, but more than 85% of the moderately prevalent pluteus transcripts are below the limit of detection of about 1 copy/cell in coelomocytes. None of the clones corresponding to rare transcripts in the pluteus is represented by prevalent transcripts in coelomocytes. The data suggest that the expression of the majority of genes giving rise to moderately prevalent messenger RNAs in embryos must be regulated during the life cycle of the sea urchin, while some very prevalent transcripts may be ubiquitous