Certain Aspergillus species such as Aspergillus flavus and A. parasiticus are well known for the formation of sclerotia. These developmental structures are thought to act as survival structures during adverse environmental conditions but are also a prerequisite for sexual reproduction. We previously described an A. niger mutant (scl-2) which formed sclerotium-like structures, suggesting a possible first stage of sexual development in this species. Several lines of evidence presented in this study support the previous conclusion that the sclerotium-like structures of scl-2 are indeed sclerotia. These included the observations that: (i) safranin staining of the sclerotia-like structures produced by the scl-2 mutant showed the typical cellular structure of a sclerotium; (ii) metabolite analysis revealed specific production of indoloterpenes, which have previously been connected to sclerotium formation; (iii) formation of the sclerotium-like structures is dependent on a functional NADPH complex, as shown for other fungi forming sclerotia. The mutation in scl-2 responsible for sclerotium formation was identified using parasexual crossing and bulk segregant analysis followed by high throughput sequencing and subsequent complementation analysis. The scl-2 strain contains a mutation that introduces a stop codon in the putative DNA binding domain of a previously uncharacterized Zn(II)2Cys6 type transcription factor (An08g07710). Targeted deletion of this transcription factor (sclB) confirmed its role as a repressor of sclerotial formation and in the promotion of asexual reproduction in A. niger. Finally, a genome-wide transcriptomic comparison of RNA extracted from sclerotia versus mycelia revealed major differences in gene expression. Induction of genes related to indoloterpene synthesis was confirmed and also let to the identification of a gene cluster essential for the production of aurasperones during sclerotium formation. Expression analysis of genes encoding other secondary metabolites, cell wall related genes, transcription factors, and genes related to reproductive processes identified many interesting candidate genes to further understand the regulation and biosynthesis of sclerotia in A. niger. The newly identified SclB transcription factor acts as a repressor of sclerotium formation and manipulation of sclB may represent a first prerequisite step towards engineering A. niger strains capable of sexual reproduction. This will provide exciting opportunities for further strain improvement in relation to protein or metabolite production in A. niger.

中文翻译：

---

SclB 的鉴定，一种参与黑曲霉菌核形成的 Zn(II)2Cys6 转录因子。

某些曲霉属物种例如黄曲霉和寄生曲霉因菌核的形成而众所周知。这些发育结构被认为是不利环境条件下的生存结构，但也是有性繁殖的先决条件。我们之前描述了一种形成菌核样结构的 A. niger 突变体 (scl-2)，表明该物种可能处于性发育的第一阶段。本研究中提供的几条证据支持先前的结论，即 scl-2 的菌核样结构确实是菌核。这些包括以下观察结果： (i) scl-2 突变体产生的菌核样结构的番红染色显示出菌核的典型细胞结构；(ii) 代谢物分析揭示了吲哚萜的特定产量，以前与菌核形成有关；(iii) 菌核样结构的形成取决于功能性 NADPH 复合物，如形成菌核的其他真菌所示。scl-2 中负责菌核形成的突变是使用准性杂交和批量分离分析确定的，然后是高通量测序和随后的互补分析。scl-2 菌株包含一个突变，该突变在先前未表征的 Zn(II)2Cys6 型转录因子 (An08g07710) 的假定 DNA 结合域中引入终止密码子。该转录因子 (sclB) 的靶向缺失证实了其作为菌核形成阻遏物和促进黑曲霉无性繁殖的作用。最后，从菌核与菌丝体中提取的 RNA 的全基因组转录组比较揭示了基因表达的主要差异。证实了与吲哚萜合成相关的基因的诱导，并且还鉴定了在菌核形成过程中产生 aurasperones 所必需的基因簇。对编码其他次级代谢产物的基因、细胞壁相关基因、转录因子和生殖过程相关基因的表达分析确定了许多有趣的候选基因，以进一步了解 A. niger 菌核的调控和生物合成。新发现的 SclB 转录因子充当菌核形成的阻遏物，操纵 sclB 可能代表了改造能够有性繁殖的黑曲霉菌株的第一个先决条件。