Abstract
Filamentous fungi of the Ascomycota phylum are known to contain a family of conserved conidiation regulating proteins with distinctive velvet domains. In Aspergilli, this velvet family includes four proteins, VeA, VelB, VelC and VosA, and is involved in conidiation and secondary metabolism along with a global regulator LaeA. In A. terreus, the overexpression of LaeA has been observed to increase the biogenesis of the pharmaceutically-important secondary metabolite, lovastatin, while the role of the velvet family has not been studied. The secondary metabolism and conidiation of A. terreus have also been observed to be increased by butyrolactone I in a quorum-sensing manner. An enlightenment of the interplay of these regulators will give potential advancement to the industrial use of this fungus, as well as in resolving the pathogenic features. In this study, the Aspergillus terreus MUCL 38669 transcriptome was strand-specifically sequenced to enable an in-depth gene expression analysis to further investigate the transcriptional role of butyrolactone I in these processes. The sequenced transcriptome revealed intriguing properties of the velvet family transcripts, including the regulator laeA, and uncovered the velC gene in A. terreus. The reliability refining microarray gene expression analysis disclosed a positive regulatory role for butyrolactone I in laeA expression, as well as an influence on the expression of the canonical conidiation-regulating genes under submerged culture. All of this supports the suggested regulative role of butyrolactone I in A. terreus secondary metabolism, as well as conidiation.
Highlights
IntroductionThe filamentous fungus Aspergillus terreus is commonly isolated from soil rhizospheres [1,2], as well as from decaying organic matter [3] and has been reported to produce a plenitude of secondary metabolites, such as itaconic acid, putative lipases and cellulases [3,4,5,6], with potential industrial importance, lovastatin with medical significance as a serum cholesterol lowering agent [7] and toxins citrinin, emodin and gliotoxin [8,9,10,11], as well as other compounds, including aspulvinones, asterriquinones, butyrolactones and (+)-geodin [12,13,14,15,16,17]
We describe the accumulation levels and alternative splicing of laeA and the velvet family transcripts veA, velB, vosA, as well as velC, whose presence in A. terreus strain MUCL 38669 and genomic locus in A. terreus strain NIH2624 was uncovered by the strand-specific whole transcriptome sequencing under the increased butyrolactone I biogenesis
The transcriptome RNA to be sequenced was obtained from the same samples used previously for whole genome gene expression study [19], from Aspergillus cultures that were exogenously fortified with butyrolactone I at 120 h post-inoculation
Summary
The filamentous fungus Aspergillus terreus is commonly isolated from soil rhizospheres [1,2], as well as from decaying organic matter [3] and has been reported to produce a plenitude of secondary metabolites, such as itaconic acid, putative lipases and cellulases [3,4,5,6], with potential industrial importance, lovastatin with medical significance as a serum cholesterol lowering agent [7] and toxins citrinin, emodin and gliotoxin [8,9,10,11], as well as other compounds, including aspulvinones, asterriquinones, butyrolactones and (+)-geodin [12,13,14,15,16,17]. Oxofuran-2-carboxylate) has been observed to enhance hyphal branching and sporulation, increase the produced amount of secondary metabolites lovastatin, sulochrin and itself and was suggested to play a quorum-sensing role in this fungus, i.e., implicating autoregulatory cell-to-cell communication in order to adapt to the environmental conditions [18,19,20]. In addition to the pharmaceutically- and industrially-important increase in secondary metabolism regarding lovastatin, the observed effect on morphological development is of significance, as well. A. terreus has been observed amongst the fungal pathogens Aspergillus fumigatus, Aspergillus flavus and Aspergillus niger to cause invasive aspergillosis in immunocompromised patients. Sporulation may play an important role in the pathogenicity by increasing the defence against diverse environmental conditions in filamentous fungi. A. terreus spores were suggested to have an important, but different role than the spores of A. fumigatus in the invasive aspergillosis by remaining vital in the macrophages after phagocytosis even in immunocompetent hosts [22]
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