As the oldest known lineage of oxygen-releasing photosynthetic organisms, cyanobacteria play the key roles in helping shaping the ecology of Earth. Iron is an ideal transition metal for redox reactions in biological systems. Cyanobacteria frequently encounter iron deficiency due to the environmental oxidation of ferrous ions to ferric ions, which are highly insoluble at physiological pH. A series of responses, including architectural changes to the photosynthetic membranes, allow cyanobacteria to withstand this condition and maintain photosynthesis. Iron-stress-induced protein A (IsiA) is homologous to the cyanobacterial chlorophyll (Chl)-binding protein, photosystem II core antenna protein CP43. IsiA is the major Chl-containing protein in iron-starved cyanobacteria, binding up to 50% of the Chl in these cells, and this Chl can be released from IsiA for the reconstruction of photosystems during the recovery from iron limitation. The pigment–protein complex (CPVI-4) encoded by isiA was identified and found to be expressed under iron-deficient conditions nearly 30years ago. However, its precise function is unknown, partially due to its complex regulation; isiA expression is induced by various types of stresses and abnormal physiological states besides iron deficiency. Furthermore, IsiA forms a range of complexes that perform different functions. In this article, we describe progress in understanding the regulation and functions of IsiA based on laboratory research using model cyanobacteria.
Summary
Cyanobacterial blooms pose a serious threat to public health due to the presence of cyanotoxins. Microcystin‐LR (MC‐LR) produced by Microcystis aeruginosa is the most common cyanotoxins. Due to the limitation of isolation, purification, and genetic manipulation techniques, it is difficult to study and verify in situ the biosynthetic pathways and molecular mechanisms of MC‐LR.
We reassembled the biosynthetic gene cluster (mcy cluster) of MC‐LR in vitro by synthetic biology, designed and constructed the strong bidirectional promoter biPpsbA2, transformed it into Synechococcus 7942, and successfully expressed MC‐LR at a level of 0.006–0.018 fg cell−1 d−1.
We found the expression of MC‐LR led to abnormal cell division and cellular filamentation, further using various methods proved that by irreversibly competing its GTP‐binding site, MC‐LR inhibits assembly of the cell division protein FtsZ.
The study represents the first reconstitution and expression of the mcy cluster and the autotrophic production of MC‐LR in model cyanobacterium, which lays the foundation for resolving the microcystins biosynthesis pathway. The discovered role of MC‐LR in cell division reveals a mechanism of how blooming cyanobacteria gain a competitive edge over their nonblooming counterparts.
Cyanobacterial blooms pose a serious threat to public health due to the presence of cyanotoxins. The most common cyanotoxins, microcystins (MCs), can cause acute poisoning at high concentrations and hepatocellular carcinoma following chronic exposure. Among all MC variants, MC-LR produced by Microcystis aeruginosa PCC 7806 is the most common toxic MC. Although the biosynthetic pathway for MC-LR has been proposed, experimental support of this pathway is lacking. In an effort to experimentally validate this pathway, we expressed the 55 kb microcystin biosynthetic gene cluster (mcy cluster) (mcyA-J) and produced MC-LR in the model cyanobacterium Synechococcus 7942. We designed and constructed the strong bidirectional promoter biPpsbA2 between mcyA and mcyD, reassembled the mcy cluster in yeast by transformation-associated recombination (TAR cloning), transformed the gene cluster into the NSII site of Synechococcus 7942, and successfully expressed MC-LR at a level of 0.006-0.018 fg cell-1 day-1. The expression of MC-LR led to abnormal cell division and the filamentation of Synechococcus 7942 cells, further analysis proved a role of MC-LR in functional assembly of the cell division protein FtsZ, by competing its GTP binding site. These results represent the first synthetic biological expression of the mcy cluster and the autotrophic production of MC-LR in a photosynthetic model organism, which lays the foundation for resolving the MC biosynthesis pathway. The suggested role of MC-LR in cell division reveals a mechanism of how blooming cyanobacteria gain a competitive edge over their non-blooming counterparts.
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