The FluG-BrlA pathway contributes to the initialisation of autolysis in submerged Aspergillus nidulans cultures Tama´s EMRI*, Zsolt MOLNA ´ R, Tu¨nde PUSZTAHELYI, Zolta´n VARECZA and Istva´n PO ´ CSI Department of Microbiology and Biotechnology, Faculty of Science, University of Debrecen, P.O. Box 63, H-4010 Debrecen, Hungary. E-mail : emri@freemail.hu Received 18 October 2004; accepted 10 April 2005. The fluG gene proved to be essential in the initialisation of autolysis in Aspergillus nidulans (teleomorph Emericella nidulans) cultures, while a loss-of-function mutation in only one out of the flbB-E genes had only minor effects on autolysis. In contrast to its important role in sporulation, brlA regulated only some, but not all, elements of the autolytic process. The tightly coupled autolytic events (chitinase and proteinase production, hyphal fragmentation, disorganisation of pellets, autolytic loss of biomass) observable in ageing cultures of A. nidulans were disconnected by loss-of-function mutations in some genes of the FluG-BrlA regulatory network. The tight correlation between pellet morphology and size and hydrolase production was also erased by these mutations. On the other hand, the mutations studied did not affect the glutathione metabolism of the fungus. INTRODUCTION Fungal autolysis is an important physiological process that needs to be tightly controlled in bioprocessing in- dustries to increase secondary metabolite yields or to prevent degradation of heterologous protein products (White et al. 2002). A deeper understanding of the underlying mechanism of fungal autolysis may also be exploited in future antifungal drug research (Thrane et al. 2004). Although the physiological and molecular basis of fungal autolysis is poorly understood, several lines of evidence support the hypothesis that this pro- cess is clearly distinguishable from necrotic cell death (Po´csi et al. 2003). For example, the physiological changes observable in carbon-depleted autolysing cultures of either Aspergillus nidulans (teleomorph Emericella nidulans) or Penicillium chrysogenum were highly concerted and regulated (Po´csi, Prade & Penninckx 2004, Emri et al. 2004). Moreover, autolysis induced by oxygen starvation in P. chrysogenum cultures was highly dependent on the availability of carbon (energy) sources suggesting that fungal autoly- sis is an energy-dependent process (McIntyre, Berry & McNeil 1999). More recently, Emri et al. (2004) re- ported that the blockage of either ATP-production by the mitochondrial uncoupler 2,4-dinitrophenol or de novo protein synthesis by cycloheximide also blocked autolysis in carbon-depleted A. nidulans cultures. The primary activator of conidiation-specific genes in A. nidulans is the transcription factor encoded by brlA (Adams, Wieser & Yu 1998, D’Souza, Lee & Adams 2001; Fig. 1). The isolation of conidiation mutants has facilitated the identification of several genes required for brlA expression. Among these genes, fadA and flbA control the balance between cell growth and sporulation (Yu, Wieser & Adams 1996, Adams et al. 1998, Rose´ n, Yu & Adams 1999; Fig. 1). The RGS (regulator of G protein signaling) domain protein FlbA is required to suppress growth signaling via FadA, the a subunit of a G protein. Concomitantly, this FlbA-dependent inhibition of FadA signaling is a prerequisite for the onset of conidiogenesis (Yu et al. 1996, Adams et al. 1998, Rose´n et al. 1999). Another major gene affecting the conidiation is fluG (Fig. 1), which encodes a cytoplasmically localised protein with homology to the prokaryotic glutamine synthetase I (Lee & Adams 1994, 1996, D’Souza et al. 2001). The hypothesised small diffusible product of FluG activity, which might be chemically related to glutamine, is thought to activate the Flb proteins. Activation of FlbA results in both the block of vegetative growth by inhibition of FadA signaling and the induction of brlA (Adams, Wieser & Yu 1998). The remaining flb genes (flbB-E) are also involved in the induction of brlA * Corresponding author. Mycol. Res. 109 (7): 757–763 (July 2005). f The British Mycological Society 757 doi:10.1017/S0953756205003023 Printed in the United Kingdom.