Light and the Transcriptional Response of the Microcystin Biosynthesis Gene Cluster

Abstract

Microcystin, a hepatotoxin known to be the cause of animal and human deaths, is produced by the bloom-forming cyanobacterium Microcystis aeruginosa in freshwater bodies worldwide. The toxin is produced nonribosomally via a multifunctional enzyme complex, consisting of both peptide synthetase and polyketide synthase modules coded for by the mcy gene cluster. The recent identification of the mcy genes in the production of microcystin synthetase for the first time provides an avenue to study the regulation of microcystin production at a genetic level. In this study, M. aeruginosa PCC7806 was grown either under continuous light of various intensities or under low light with subsequent short-term exposure to different light intensities and qualities and various stress factors. RNase protection assays were employed to observe the level of mcyB and mcyD transcription under each condition. Both mcyB and mcyD transcript levels were increased under high light intensities and red light. Blue light and certain artificial stress factors (methylviologen and NaCl) led to reduced transcript amounts. There appeared to be two light thresholds, between dark and low light (16 mol of photons m ؊2 s ؊1 ), and medium (31 mol of photons m ؊2 s ؊1 ) and high light (68 mol of photons m ؊2 s ؊1 ), at which a significant increase in transcription occurred. Our findings show that the effect of light on microcystin synthetase production is due to light quality and is initiated at certain threshold intensities, which are not necessarily reflected by observed intracellular toxin bioactivity.

Altered expression of two light-dependent genes in a microcystin-lacking mutant of Microcystis aeruginosa PCC 7806

“…So far, microcystin detected outside the cells in the culture medium, under high light, has largely been attributed to cell lysis (Rapala et al, 1997). Yet this may also suggest an export, and point to an extracellular role for microcystin under these conditions (Kaebernick et al, 2000 ;E. Dittmann & M. Kaebernick, unpublished data).…”

“…However, with the inability to compare results from various studies due to different culturing and analysis methods used, no conclusions can be made about the effect of growth rate or environmental factors regulating toxin production. More recently, molecular techniques have been employed to show an effect of light on microcystin biogenesis in two strains of M. aeruginosa (Nishizawa et al, 1999 ;Kaebernick et al, 2000).…”

Microbial communities reflect temporal changes in cyanobacterial composition in a shallow ephemeral freshwater lake

“…There is an evident difficulty in correlating many environmental parameters with cyanobacterial community composition, toxicity or toxigenicity across such studies (Rinta-Kanto et al, 2009;Al-Tebrineh et al, 2011;Otten et al, 2012;Lee et al, 2014;Ngwa et al, 2014). In part, this lack of correlation can be attributed to genomic variability among closely related strains (Humbert et al, 2013;Sinha et al, 2014), giving rise to differing growth optima and potential to produce a myriad of toxic and non-toxic metabolites (Humbert et al, 2013), changes in the genome copy number throughout the growth phase (Griese et al, 2011) and uncertainty regarding the control of regulatory systems that direct the expression of toxic metabolites (Kaebernick et al, 2000;Alexova et al, 2011;Carneiro et al, 2013;Neilan et al, 2013;Rzymski and Poniedziałek, 2014;Makower et al, 2015).…”

Towards clarification of the inhibitory mechanism of wheat bran leachate on Microcystis aeruginosa NIES-843 (cyanobacteria): physiological responses

“…Recently, the influence of environmental factors on microcystins synthesis was studied in levels of transcription of mcy genes and microcystins synthetase (Mcy). Kaebernick et al (2000) found that transcript levels of both mcyB and mcyD up-regulated under high light intensities and red light. Schatz et al (2007) found cellular lysis of Microcystis induced massive accumulation of McyB and enhanced the production of microcystins in the remaining Microcystis cells, and therefore proposed that releasing microcystins from lytic cells act as info-chemicals to stimulate microcystins biosynthesis and thereby enhancing their fitness in their ecological niche.…”