Assuntos
Toxinas Bacterianas/genética , Toxinas Bacterianas/toxicidade , Cianobactérias/genética , Cianobactérias/patogenicidade , Eutrofização , Toxinas Marinhas/genética , Toxinas Marinhas/toxicidade , Microcistinas/genética , Microcistinas/toxicidade , Alcaloides , Animais , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Toxinas Bacterianas/biossíntese , Evolução Biológica , Cianobactérias/metabolismo , Toxinas de Cianobactérias , Água Doce/análise , Água Doce/microbiologia , Expressão Gênica , Genes Bacterianos , Genômica , Humanos , Toxinas Marinhas/biossíntese , Microcistinas/biossíntese , Peptídeo Sintases/genética , Peptídeo Sintases/metabolismo , Peptídeos Cíclicos/biossíntese , Peptídeos Cíclicos/genética , Peptídeos Cíclicos/toxicidade , Saxitoxina/biossíntese , Saxitoxina/genética , Saxitoxina/toxicidade , Tropanos/toxicidade , Uracila/análogos & derivados , Uracila/biossíntese , Uracila/toxicidadeRESUMO
Microcystin is a potent inhibitor of eukaryotic protein phosphatases and has been implicated in causing hepatotoxicity to humans and animals worldwide. It is produced primarily by the bloom-forming cyanobacterium Microcystis aeruginosa, although the function of the peptide in this micro-organism is unknown. In this study, a microcystin-related protein, MrpA, was identified using a microcystin-lacking mutant of M. aeruginosa, PCC 7806. Comparative two-dimensional protein electrophoresis showed that MrpA was strongly expressed in wild-type PCC 7806, but was not detectable in the mcyB mutant. MrpA showed similarity to the RhiA protein from Rhizobium leguminosarum, which is encoded by the rhiABC operon and controlled by quorum-sensing mediators. Sequencing of mrpA flanking regions in M. aeruginosa PCC 7806 revealed the presence of a rhiB homologue, mrpB, directly downstream of mrpA. Northern blot analyses of mrpA expression in cells exposed to different light conditions revealed a rapid decline of transcription under high light conditions. Most striking was a strong increase in transcript levels from cultures irradiated with blue light. The mrpA transcription level was strongly reduced in two independent microcystin-lacking mutants under all light conditions investigated.
Assuntos
Proteínas de Algas/genética , Regulação Bacteriana da Expressão Gênica , Microcystis/enzimologia , Peptídeos Cíclicos/fisiologia , Proteínas de Algas/química , Sequência de Aminoácidos , Proteínas de Bactérias/química , Luz , Microcistinas , Microcystis/genética , Microcystis/crescimento & desenvolvimento , Dados de Sequência Molecular , Complexos Multienzimáticos , Família Multigênica , Peptídeos Cíclicos/genética , RNA Bacteriano/genética , RNA Bacteriano/metabolismo , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Rhizobium leguminosarum/química , Transcrição GênicaRESUMO
Microcystis aeruginosa strain MRC is unique in its' possession of the mcyA-J gene cluster, which encodes microcystin synthetase, but its' inability to produce microcystins. M. aeruginosa strain MRD is genetically identical to MRC at numerous genomic loci examined, but produces a variety of microcystins, mainly with the amino acid tyrosine in the molecule. Zooplankton studies with Daphnia galeata and D. pulicaria, using the mutant (MRC) and its' wild type (MRD), showed for the first time that microcystins other than microcystin-LR can be responsible for the poisoning of Daphnia by Microcystis. Regardless of microcystin content, both Daphnia exhibited significantly reduced ingestion rates when fed with either strain of M. aeruginosa compared with the green alga Scenedesmus acutus. A disruption of the molting process in both Daphnia spp. was noted when these species were fed with MRC cells. Such symptoms on Daphnia have not been previously reported for cyanobacteria and may point to a bioactive compound, other than microcystin, which inhibits the hardening of protein-chitin complexes in Daphnia.
Assuntos
Cianobactérias/genética , Daphnia/fisiologia , Peptídeo Sintases/genética , Peptídeos Cíclicos/biossíntese , Peptídeos Cíclicos/toxicidade , Animais , Proteínas de Bactérias , Toxinas Bacterianas/biossíntese , Toxinas Bacterianas/genética , Toxinas Bacterianas/toxicidade , Cromatografia Líquida de Alta Pressão , Cianobactérias/enzimologia , Dieta , Inibidores Enzimáticos/metabolismo , Inibidores Enzimáticos/toxicidade , Humanos , Microcistinas , Peptídeos Cíclicos/genética , Espectrometria de Massas por Ionização e Dessorção a Laser Assistida por Matriz , Taxa de SobrevidaRESUMO
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 micromol of photons m(-2) s(-1)), and medium (31 micromol of photons m(-2) s(-1)) and high light (68 micromol 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.