[Synthesis of carbazole derivatives. 1. The reaction of 3-(2-nitroethenyl)-indole-2-malonic acid esters with Michael acceptors]

Michael-type reaction of compounds of type 9 were studied. Addition reactions with acrolein and methyl vinyl ketone, respectively, lead--depending on the conditions--to pyridoindoles of type 16 and 14 and/or to tetrahydrocarbazoles of type 17 and 15. With Triton B compounds 15 undergo retro-Michael-type elimination of the nitromethyl group, followed by hydrolysis/decarboxylation and dehydrogenation, leading to the disubstituted carbazoles 20. In the case of 17a, the primary elimination product 18 could be isolated. The synthetic potential of these reaction types is discussed. Some of the described compounds and derivatives of them show antimycobacterial activity.

RimO (SrrB) is required for carbon starvation signaling and production of secondary metabolites in Aspergillus nidulans

Depending on the prevailing environmental, developmental and nutritional conditions, fungi activate biosynthetic gene clusters (BGCs) to produce condition-specific secondary metabolites (SMs). For activation, global chromatin-based de-repression must be integrated with pathway-specific induction signals. Here we describe a new global regulator needed to activate starvation-induced SMs. In our transcriptome dataset, we found locus AN7572 strongly transcribed solely under conditions of starvation-induced SM production. The predicted AN7572 protein is most similar to the stress and nutritional regulator Rim15 of Saccharomyces cerevisiae, and to STK-12 of Neurospora crassa. Based on this similarity and on stress and nutritional response phenotypes of A. nidulans knock-out and overexpression strains, AN7572 is designated rimO. In relation to SM production, we found that RimO is required for the activation of starvation-induced BGCs, including the sterigmatocystin (ST) gene cluster. Here, RimO regulates the pathway-specific transcription factor AflR both at the transcriptional and post-translational level. At the transcriptional level, RimO mediates aflR induction following carbon starvation and at the post-translational level, RimO is required for nuclear accumulation of the AflR protein. Genome-wide transcriptional profiling showed that cells lacking rimO fail to adapt to carbon starvation that, in the wild type, leads to down-regulation of genes involved in basic metabolism, membrane biogenesis and growth. Consistently, strains overexpressing rimO are more resistant to oxidative and osmotic stress, largely insensitive to glucose repression and strongly overproduce several SMs. Our data indicate that RimO is a positive regulator within the SM and stress response network, but this requires nutrient depletion that triggers both, rimO gene transcription and activation of the RimO protein.

Transcriptional memory drives accelerated re-activation of several biosynthetic gene clusters in Aspergillus nidulans

Organisms are repeatedly exposed to fluctuating environmental and nutritional conditions. Transcriptional memory has been shown to be a mechanism to cope with these fluctuations because it increases the speed and the magnitude of the cellular response to a certain re-occurring condition and therefore optimizes adaptation and fitness in a given environment. We found that genes coding for sterigmatocystin (ST) production in Aspergillus nidulans are activated stronger when cells are repeatedly exposed to nutrient starvation, compared to cells that experience this condition for the first time. We studied possible underlying mechanisms and found that persistence of the transcription factor AflR, which can undergo activation-inactivation cycles, accounts for a large part of the memory. In addition, a chromatin-based mechanism through histone H3 lysine 4 dimethylation (H3K4me2) and extracellular metabolites produced during the first activation phase contribute to the memory process. Genome-wide transcriptome and chromatin analyses showed that only a few genes within the ST and other starvation-induced biosynthetic gene clusters gain the H3K4me2 mark during the 1st activation, but the majority of those which receive the mark also maintain it during the subsequent repression and re-activation phase. Combined with previous findings on chromatin-level regulation of biosynthetic gene clusters (BGCs) our recent data suggest that the H3K4me2 mark may contribute to the correct 3D organization of BGCs and that this is a prerequisite for activation and transcriptional memory.