In the facultative autotrophic organism Chloroflexus aurantiacus, a phototrophic green nonsulfur bacterium, the Calvin cycle does not appear to be operative in autotrophic carbon assimilation. An alternative cyclic pathway, the 3-hydroxypropionate cycle, has been proposed. In this pathway, acetyl coenzyme A (acetyl-CoA) is assumed to be converted to malate, and two CO 2 molecules are thereby fixed. Malyl-CoA is supposed to be cleaved to acetyl-CoA, the starting molecule, and glyoxylate, the carbon fixation product. Malyl-CoA cleavage is shown here to be catalyzed by malyl-CoA lyase; this enzyme activity is induced severalfold in autotrophically grown cells. Malate is converted to malyl-CoA via an inducible CoA transferase with succinyl-CoA as a CoA donor. Some enzyme activities involved in the conversion of malonyl-CoA via 3-hydroxypropionate to propionyl-CoA are also induced under autotrophic growth conditions. So far, no clue as to the first step in glyoxylate assimilation has been obtained. One possibility for the assimilation of glyoxylate involves the conversion of glyoxylate to glycine and the subsequent assimilation of glycine. However, such a pathway does not occur, as shown by labeling of whole cells with [1,2-13 C 2 ]glycine. Glycine carbon was incorporated only into glycine, serine, and compounds that contained C 1 units derived therefrom and not into other cell compounds.
The Ustilago maydis mig2 cluster comprises five highly homologous genes that display a pronounced plant-specific expression profile. A 350-bp mig2-5 promoter fragment contained all elements sufficient to confer differential promoter activity. Mutational analysis of this region, fused to the green fluorescent protein reporter gene, allowed dissecting core promoter elements required for high-level promoter activity from elements conferring inducible expression in planta. In particular, the presence of several 5'-CCA-3' motifs within a short stretch of the mig2-5 promoter was decisive for inducible promoter activity. On this basis, we reconstituted an artificial promoter whose inducible activity specifically relied on multiple CCA motifs. In addition, we identified a novel mig2 homologous gene, mig2-6, that is not part of the mig2 cluster, but displayed the strongest differential expression profile among mig2 genes. The deletion of all six mig2 genes did not compromise the ability to induce tumor formation in infected maize plants. Comparative sequence analysis including the mig2-6 promoter revealed an over-representation of the consensus motif 5'-MNMNWNCCAMM-3'. We discuss putative transcriptional activators involved in mig2 regulation.
SummaryThe smut fungus Ustilago maydis establishes a biotrophic relationship with its host plant maize to progress through sexual development. Here, we report the identification and characterization of the Cys2His2-type zinc finger protein Mzr1 that functions as a transcriptional activator during host colonization. Expression of the U. maydis mig2 cluster genes is tightly linked to this phase. Upon conditional overexpression, Mzr1 confers induction of a subset of mig2 genes during vegetative growth and this requires the same promoter elements that confer inducible expression in planta. Furthermore, expression of the mig2-4 and mig2-5 genes during biotrophic growth is strongly reduced in cells deleted in mzr1. DNA-array analysis led to the identification of additional Mzr1-induced genes. Some of these genes show a mig2-like plantspecific expression pattern and Mzr1 is responsible for their high-level expression during pathogenesis. Mzr1 function requires the b-dependently regulated Cys 2His2-type cell cycle regulator Biz1, indicating that two stage-specific regulators mediate gene expression during host colonization. In spite of a role as transcriptional activator during biotrophic growth, mzr1 is not essential for pathogenesis; however, conditional overexpression interfered with proliferation during vegetative growth and mating ability, caused a cell separation defect, and triggered filamentous growth. We discuss the implications of these findings.
The lifestyle of phytopathogenic fungi is strongly determined by their environment. This implies that mechanisms providing for versatile gene regulation in response to external signals or during host associations exist. In Ustilago maydis, central players of pathogenic development are the high mobility group box protein Prf1 that binds to the pheromone response element and the homeodomain transcription factor b, which recognizes an hsg-like consensus motif known from yeast Mata1-Matalpha2 DNA binding. Transcription of prf1 is influenced by multiple inputs and this is reflected by its modular promoter structure. Analysis of the U. maydis mig promoters provides a link to transcriptional regulation during biotrophic growth. Furthermore, recognition of repeated GATA sequences as well as of triplet motifs by transcription factors with binuclear Zn(II)(2)Cys(6) DNA-binding domains appears to mediate diverse transcriptional responses relevant for phytopathogenic fungi. Although present studies shed some light on the complexity of transcriptional processes operating in phytopathogenic fungi, further investigation of promoter structures including the involvement of ubiquitous promoter elements is needed. Confronted with increasing genome-wide analysis, knowledge of promoter structures not only allows predicting transcriptional regulation, but might also advance our understanding of transcriptional networks.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.