39 research outputs found
Origin of Saxitoxin Biosynthetic Genes in Cyanobacteria
BACKGROUND:Paralytic shellfish poisoning (PSP) is a potentially fatal syndrome associated with the consumption of shellfish that have accumulated saxitoxin (STX). STX is produced by microscopic marine dinoflagellate algae. Little is known about the origin and spread of saxitoxin genes in these under-studied eukaryotes. Fortuitously, some freshwater cyanobacteria also produce STX, providing an ideal model for studying its biosynthesis. Here we focus on saxitoxin-producing cyanobacteria and their non-toxic sisters to elucidate the origin of genes involved in the putative STX biosynthetic pathway. METHODOLOGY/PRINCIPAL FINDINGS:We generated a draft genome assembly of the saxitoxin-producing (STX+) cyanobacterium Anabaena circinalis ACBU02 and searched for 26 candidate saxitoxin-genes (named sxtA to sxtZ) that were recently identified in the toxic strain Cylindrospermopsis raciborskii T3. We also generated a draft assembly of the non-toxic (STX-) sister Anabaena circinalis ACFR02 to aid the identification of saxitoxin-specific genes. Comparative phylogenomic analyses revealed that nine putative STX genes were horizontally transferred from non-cyanobacterial sources, whereas one key gene (sxtA) originated in STX+ cyanobacteria via two independent horizontal transfers followed by fusion. In total, of the 26 candidate saxitoxin-genes, 13 are of cyanobacterial provenance and are monophyletic among the STX+ taxa, four are shared amongst STX+ and STX-cyanobacteria, and the remaining nine genes are specific to STX+ cyanobacteria. CONCLUSIONS/SIGNIFICANCE:Our results provide evidence that the assembly of STX genes in ACBU02 involved multiple HGT events from different sources followed presumably by coordination of the expression of foreign and native genes in the common ancestor of STX+ cyanobacteria. The ability to produce saxitoxin was subsequently lost multiple independent times resulting in a nested relationship of STX+ and STX- strains among Anabaena circinalis strains
Discovery of Nuclear-Encoded Genes for the Neurotoxin Saxitoxin in Dinoflagellates
Saxitoxin is a potent neurotoxin that occurs in aquatic environments worldwide.
Ingestion of vector species can lead to paralytic shellfish poisoning, a severe
human illness that may lead to paralysis and death. In freshwaters, the toxin is
produced by prokaryotic cyanobacteria; in marine waters, it is associated with
eukaryotic dinoflagellates. However, several studies suggest that saxitoxin is
not produced by dinoflagellates themselves, but by co-cultured bacteria. Here,
we show that genes required for saxitoxin synthesis are encoded in the nuclear
genomes of dinoflagellates. We sequenced >1.2×106 mRNA
transcripts from the two saxitoxin-producing dinoflagellate strains
Alexandrium fundyense CCMP1719 and A.
minutum CCMP113 using high-throughput sequencing technology. In
addition, we used in silico transcriptome analyses, RACE, qPCR
and conventional PCR coupled with Sanger sequencing. These approaches
successfully identified genes required for saxitoxin-synthesis in the two
transcriptomes. We focused on sxtA, the unique starting gene of
saxitoxin synthesis, and show that the dinoflagellate transcripts of
sxtA have the same domain structure as the cyanobacterial
sxtA genes. But, in contrast to the bacterial homologs, the
dinoflagellate transcripts are monocistronic, have a higher GC content, occur in
multiple copies, contain typical dinoflagellate spliced-leader sequences and
eukaryotic polyA-tails. Further, we investigated 28 saxitoxin-producing and
non-producing dinoflagellate strains from six different genera for the presence
of genomic sxtA homologs. Our results show very good agreement
between the presence of sxtA and saxitoxin-synthesis, except in
three strains of A. tamarense, for which we amplified
sxtA, but did not detect the toxin. Our work opens for
possibilities to develop molecular tools to detect saxitoxin-producing
dinoflagellates in the environment
Transcriptomic response of the red tide dinoflagellate, Karenia brevis, to nitrogen and phosphorus depletion and addition
<p>Abstract</p> <p>Background</p> <p>The role of coastal nutrient sources in the persistence of <it>Karenia brevis </it>red tides in coastal waters of Florida is a contentious issue that warrants investigation into the regulation of nutrient responses in this dinoflagellate. In other phytoplankton studied, nutrient status is reflected by the expression levels of N- and P-responsive gene transcripts. In dinoflagellates, however, many processes are regulated post-transcriptionally. All nuclear encoded gene transcripts studied to date possess a 5' <it>trans</it>-spliced leader (SL) sequence suggestive, based on the trypanosome model, of post-transcriptional regulation. The current study therefore sought to determine if the transcriptome of <it>K. brevis </it>is responsive to nitrogen and phosphorus and is informative of nutrient status.</p> <p>Results</p> <p>Microarray analysis of N-depleted <it>K. brevis </it>cultures revealed an increase in the expression of transcripts involved in N-assimilation (nitrate and ammonium transporters, glutamine synthetases) relative to nutrient replete cells. In contrast, a transcriptional signal of P-starvation was not apparent despite evidence of P-starvation based on their rapid growth response to P-addition. To study transcriptome responses to nutrient addition, the limiting nutrient was added to depleted cells and changes in global gene expression were assessed over the first 48 hours following nutrient addition. Both N- and P-addition resulted in significant changes in approximately 4% of genes on the microarray, using a significance cutoff of 1.7-fold and p ≤ 10<sup>-4</sup>. By far, the earliest responding genes were dominated in both nutrient treatments by pentatricopeptide repeat (PPR) proteins, which increased in expression up to 3-fold by 1 h following nutrient addition. PPR proteins are nuclear encoded proteins involved in chloroplast and mitochondria RNA processing. Correspondingly, other functions enriched in response to both nutrients were photosystem and ribosomal genes.</p> <p>Conclusions</p> <p>Microarray analysis provided transcriptomic evidence for N- but not P-limitation in <it>K. brevis</it>. Transcriptomic responses to the addition of either N or P suggest a concerted program leading to the reactivation of chloroplast functions. Even the earliest responding PPR protein transcripts possess a 5' SL sequence that suggests post-transcriptional control. Given the current state of knowledge of dinoflagellate gene regulation, it is currently unclear how these rapid changes in such transcript levels are achieved.</p
Effect of panicle removal on cytokinin level in the xylem and nitrogen uptake activity of rice
To evaluate the role of cytokinin in the source–sink relationship, panicles of rice were cut from the stem at the panicle emergence stage. Xylem sap exudates were collected using the stem cut method and the cytokinin concentration in the collected sap was determined by bioassay and further analysis using enzyme-linked immunosorbent assay. The rate of cytokinin translocation from roots to shoots decreased continuously after panicle initiation, whereas, when the panicle was removed, the rate increased by up to 1.5-fold, at which time no cytokinin was found in the plants with panicles. Retardation of leaf senescence was not observed and nitrogen concentration in the leaves continued to decrease after panicle removal, irrespective of cytokinin (mainly dihydrozeatin riboside and trans-zeatin riboside) level. Thus, leaf autonomy is regulated by an endogenous program of nitrogen translocation from the leaf regardless of cytokinin level in the xylem