Studies on the sexual development of Aspergillus nidulans

Abstract

Beside the linker histones (H1 and H5), the B-type high mobility group box domain proteins (HMGB), as architectural components of the chromatin, participate in various functions of chromatin, such as recombination, repair, inducing or repressing gene expressions. They are able to interact with both DNA and protein components of the chromatin through their high mobility group box (HMG-box) domains. Amongst the fungal HMGB proteins, role of the yeast HMGB proteins was studied in detail, providing vast knowledge on the mode-of-action, molecular- and physiological role of HMGB proteins. However, very few data is known aboult the role of HMGB proteins of filamentous fungi. Detailed study on the function of mitochondrial HMGB protein of P. anserina (mtHMG1) and A. nidulans (HmbB) revealed their role in the maintenance of mitochondrial genome and additionally, nuclear roles were also proposed. Besides the mitochondrial HMGB proteins, four nuclear HMGB proteins were identified in P. anserina and two in A. nidulans (HmbA and HmbC). A recent study on the HMGB proteins of P. anserina revealed that they are involved in the sexual development and fruiting body formation of the fungus. Two of them, PaHMG6 and mtHMG1, also play role in the positive and negative regulation of the expression of the mating-type ranscription factors, respectively. In this work we aimed to study the role of A. nidulans HMGB proteins in the sexual development and found that some of their functions are specific. Through the study of sexual development of control and hmbA, hmbB and hmbC deletion mutants both in veA+ and veA1 genetic background we revealed that HmbA, HmbB and HmbC architectural chromatin components of A. nidulans are required for normal sexual development, especially for the formation and viability of ascospores and the spatial distribution of cleistothecia. Furthermore, we proposed that HmbC functionally interacts with VeA, the key reulator of development and secondary production. We suggested that HmbA and HmbB might be involved in sensing of and responding to the changes of environmental oxygen levels. All HMGB proteins, HmbA, HmbB and HmbC were found to be critical for the normal expression of the mating-type transcription factor genes. We excluded the possibility that all three HMGB linker proteins are required for the direct activation of MAT gene expression. Instead, we propose an alternative explanation for the observed changes: HmbA, HmbB and HmbC might operate on upstream regulator(s) of MAT genes that is (are) most probably involved in the sensing of environmental and/or intracellular factors and/or the transduction of related signals that affect the activation of the MAT genes. Future research should elucidate the potential role of HMGB proteins in the upstream regulation of MAT genes. On the other hand, despite the uniform downregulation of the mating type transcription genes in the hmbA, hmbB and hmbC deletion mutants, we found that only the hmbA deletion phenocopied the mating type gene deletion phenotypes, hmbB and hmbC deletions resulted in various levels of defects in ascospore production and ascospore viability. As an explanation for the various levels of defects observed in hmbB and hmbC deletion mutants we proposed that HmbA, HmbB and HmbC might directly influence the gene expression of various MAT-regulated genes. Additionally, we suppose that hmbA, hmbB and hmbC can provide the functional backups for each other’s loss. Considering all of the phenotypes of the hmbA, hmbB and hmbC strains, and the transcriptional changes observed in case of the expression of sexual development involved regulators we supposed that besides the pronounced down-regulation of MAT genes, a qualitatively different perturbation in the expression of other genes might further contribute to the hmbA-, hmbB- and hmbC-specific phenotypes. Some of the physiological functions of HMGB proteins we revealed are specific for A. nidulans compared to yeast and P. anserina. This includes that HmbA and HmbB play a role in sensing of and/or response to environmental signals. By revealing the functional connections of HmbA and HmbB with signal transduction pathways, one would gain a deeper understanding of the regulatory machinery that governs physiological responses to environmental changes. On the other hand, we found that HmbC functionally interacts with VeA, a key regulator of the coordination of asexual and sexual development, as well as of secondary metabolism. By revealing the functional interactions of HmbC, one would gain a deeper insight into the regulation of these biological processes. Finally, HmbA, HmbB and HmbC are equally important in the positive regulation of mating-type genes, and thereby have a great impact on ascospore production in A. nidulans.