Melanisation of Aspergillus terreus—Is Butyrolactone I Involved in the Regulation of Both DOPA and DHN Types of Pigments in Submerged Culture?

Abstract

Pigments and melanins of fungal spores have been investigated for decades, revealing important roles in the survival of the fungus in hostile environments. The key genes and the encoded enzymes for pigment and melanin biosynthesis have recently been found in Ascomycota, including Aspergillus spp. In Aspergillus terreus, the pigmentation has remained mysterious with only one class of melanin biogenesis being found. In this study, we examined an intriguing, partially annotated gene cluster of A. terreus strain NIH2624, utilizing previously sequenced transcriptome and improved gene expression data of strain MUCL 38669, under the influence of a suggested quorum sensing inducing metabolite, butyrolactone I. The core polyketide synthase (PKS) gene of the cluster was predicted to be significantly longer on the basis of the obtained transcriptional data, and the surrounding cluster was positively regulated by butyrolactone I at the late growth phase of submerged culture, presumably during sporulation. Phylogenetic analysis of the extended PKS revealed remarkable similarity with a group of known pigments of Fusarium spp., indicating a similar function for this PKS. We present a hypothesis of this PKS cluster to biosynthesise a 1,8-dihydroxynaphthalene (DHN)-type of pigment during sporulation with the influence of butyrolactone I under submerged culture.

Reply to the Comment on “Melanisation of Aspergillus terreus—Is Butyrolactone I Involved in the Regulation of Both DOPA and DHN Types of Pigments in Submerged Culture? Microorganisms 2017, 5, 22”

“…Regarding the several further thoughts presented by Geib and Brock [ 3 ], we agree on the necessity to confirm them with further studies. In our previous study [ 4 ], we observed an increase in lovastatin production, both on the transcriptional and molecular level, and showed it to be enhanced already 24 h after butyrolactone I supplementation, 96 h prior to the speculated pgm cluster genes showed upregulation as described in our current study [ 5 ]. Similarly, in our recent study [ 6 ], we observed the global regulator gene laeA to be upregulated 24 h after butyrolactone I supplementation, i.e., 96 h prior to the speculated pgm cluster genes.…”

“…Schimmel et al [ 8 ] reported butyrolactone I to increase the number of spores as well as lovastatin production. The studies of Raina et al, Palonen et al and Schimmel et al [ 4 , 5 , 6 , 8 ] were performed in submerged and continuously shaken culture conditions, although we have no specific morphological information regarding sporulation. Therefore, we do agree that none of these observations define the function of this pgm gene cluster to be specifically related to pigmentation but a difference with the occurred secondary metabolism was hypothesised.…”

“…As pointed out by Geib and Brock [ 3 ], the observed similarity with the fusarubin gene cluster of the Fusarium fujikuroi may indicate sexual development during the growth conditions used in this study. This raises a question regarding the observed upregulation of one of the key regulators of asexual conidiation, abaA —occurring exactly at the same time point as the pgm cluster genes are upregulated as well [ 5 , 6 ]. Some of the regulative genes of sexual development were also expressed under the applied growth conditions (data not shown).…”

Comment on: “Melanisation of Aspergillus terreus—Is Butyrolactone I Involved in the Regulation of Both DOPA and DHN Types of Pigments in Submerged Culture? Microorganisms 2017, 5, 22”

“…Palonen et al [1] used a submerged culture condition throughout their studies in which they supplemented the medium artificially with butyrolactone I. Under these conditions, an increased expression of the pgm gene cluster accompanied by a decreasing expression of the Asp-melanin gene cluster was observed.…”

In silico Prediction, Characterization, Molecular Docking, and Dynamic Studies on Fungal SDRs as Novel Targets for Searching Potential Fungicides Against Fusarium Wilt in Tomato

“…During the biosynthesis of fungal melanin through pentaketide pathway, tetrahydroxynaphthalene reductase (T4HNR) catalyzes the NADP(H)-dependent reduction of 1,3,6,8-tetrahydroxynaphthalene (THN) into (+)-scytalone and 1,3,8-trihydroxynaphthalene into (−)-vermelone (Figure 1 ). The DHN pathway-based classification depends on their preference for the Naphthoquinon precursors or on the effect of inhibitors such as phthalide or tricyclazole, which binds with hydroxynaphthalene reductases having classical short-chain dehydrogenase/reductase (SDR) with Rossmann-fold domains ( Palonen et al, 2017 ). It has been reported that the polyketide pathway in filamentous fungi is an important metabolic process that regulates their growth, development, and pathogenicity ( Xiong et al, 2014 ).…”