Abstract
Transcription factors (TFs) containing the basic leucine zipper (bZIP) domain are widely distributed in eukaryotes and display an array of distinct functions. In this study, a bZIP-type TF gene (MBZ1) was deleted and functionally characterized in the insect pathogenic fungus Metarhizium robertsii. The deletion mutant (ΔMBZ1) showed defects in cell wall integrity, adhesion to hydrophobic surfaces, and topical infection of insects. Relative to the WT, ΔMBZ1 was also impaired in growth and conidiogenesis. Examination of putative target gene expression indicated that the genes involved in chitin biosynthesis were differentially transcribed in ΔMBZ1 compared with the WT, which led to the accumulation of a higher level of chitin in mutant cell walls. MBZ1 exhibited negative regulation of subtilisin proteases, but positive control of an adhesin gene, which is consistent with the observation of effects on cell autolysis and a reduction in spore adherence to hydrophobic surfaces in ΔMBZ1. Promoter binding assays indicated that MBZ1 can bind to different target genes and suggested the possibility of heterodimer formation to increase the diversity of the MBZ1 regulatory network. The results of this study advance our understanding of the divergence of bZIP-type TFs at both intra- and interspecific levels.
Highlights
The basic leucine zipper (bZIP)-type transcription factors are widely distributed in eukaryotes to control an array of biological activities
Characteristics of MBZ1—Genome-wide analysis indicated that there are 24 bZIP domain-containing Transcription factors (TFs) encoded in the genome of M. robertsii, whereas 14 –26 bZIP-type TFs are present in yeast and other fungi (Fig. 1A)
Subfamily 1 contains the yeast proteins CST6, ACA1, and SKO1 (Fig. 1B), which belong to the cAMP response element-binding protein (CREB) family of bZIP-type TFs that bind the consensus palindromic site 5Ј-TGACGTCA-3Ј [41]
Summary
The bZIP-type transcription factors are widely distributed in eukaryotes to control an array of biological activities. A bZIP-type TF gene (MBZ1) was deleted and functionally characterized in the insect pathogenic fungus Metarhizium robertsii. Deletions and functional characterizations of these genes revealed that bZIP-type TFs are involved in mediating fungal development, sexuality, stress responses, secondary metabolisms, and especially the virulence of plant pathogens. Genome analyses of insect fungal pathogens such as Metarhizium robertsii [16], Cordyceps militaris [17], and Beauveria bassiana [18] identified an array of bZIP-type TFs present in their genomes None of these has been functionally studied. Our functional studies showed that MBZ1 mediates both positive and negative regulation of different target genes involved in growth, sporulation, cell wall integrity, spore adherence, and virulence against insect hosts in M. robertsii
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