A filamentous non-N2-fixing cyanobacterium, Arthrospira (Spirulina) platensis, is an important organism for industrial applications and as a food supply. Almost the complete genome of A. platensis NIES-39 was determined in this study. The genome structure of A. platensis is estimated to be a single, circular chromosome of 6.8 Mb, based on optical mapping. Annotation of this 6.7 Mb sequence yielded 6630 protein-coding genes as well as two sets of rRNA genes and 40 tRNA genes. Of the protein-coding genes, 78% are similar to those of other organisms; the remaining 22% are currently unknown. A total 612 kb of the genome comprise group II introns, insertion sequences and some repetitive elements. Group I introns are located in a protein-coding region. Abundant restriction-modification systems were determined. Unique features in the gene composition were noted, particularly in a large number of genes for adenylate cyclase and haemolysin-like Ca2+-binding proteins and in chemotaxis proteins. Filament-specific genes were highlighted by comparative genomic analysis.
Summary
The target genes for SYCRP1, a cyanobacterial cAMP receptor protein, were surveyed using a DNA micro‐array method. Total RNAs were extracted from a wild‐type strain and a sycrp1 disruptant of Synechocystis sp. PCC 6803, and the respective
gene expression levels were compared. The expression levels of six genes (slr1667,
slr1668, slr2015, slr2016, slr2017 and slr2018) were clearly decreased by the disruption
of the sycrp1 gene. The data suggest that slr1667 and slr1668 constitute one
operon and the other four genes constitute another operon. Transcription start points
for the first genes of these putative operons, which are slr1667 and slr2015, were
determined by primer extension experiments. Gel mobility shift assays and DNase I
footprint analyses were carried out to explore the binding of SYCRP1 to the putative
promoter regions of slr1667 and slr2015. SYCRP1 bound to the specific site in the
5′ upstream region of slr1667 from positions –170 to –155
relative to the transcription start point, while it did not bind to the 5′ upstream region of slr2015. It was concluded that SYCRP1 regulates the expression of the slr1667 gene directly by binding to a specific site in its promoter.
Three open reading frames of Synechocystis sp. PCC 6803 encoding a domain homologous with the cAMP binding domain of bacterial cAMP receptor protein were analyzed. These three open reading frames, sll1371, sll1924, and slr0593, which were named sycrp1, sycrp2, and sypk, respectively, were expressed in Escherichia coli as His-tagged or glutathione S-transferase fusion proteins and purified, and their biochemical properties were investigated. The results obtained for equilibrium dialysis measurements using these recombinant proteins suggest that SYCRP1 and SYPK show a binding affinity for cAMP while SYCRP2 does not. The dissociation constant of His-tagged SYCRP1 for cAMP is approximately 3 M. A cross-linking experiment using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide revealed that His-tagged SYCRP1 forms a homodimer, and the presence or absence of cAMP does not affect the formation of the homodimer. The amino acid sequence reveals that SYCRP1 has a domain similar to the DNA binding domain of bacterial cAMP receptor protein in the COOH-terminal region. Consistent with this, His-tagged SYCRP1 forms a complex with DNA that contains the consensus sequence for E. coli cAMP receptor protein in the presence of cAMP. These results strongly suggest that SYCRP1 is a novel cAMP receptor protein.
Disruption of the sycrp1 gene encoding a cyanobacterial cAMP receptor protein makes cells of Synechocystis sp. PCC 6803 non-motile. Electron microscopy showed that the sycrp1-deficient strain had a reduced number of thick pili on the cell surface compared with the wild-type strain. It is suggested that cAMP-SYCRP1 complex controls the biogenesis of pili.
The changes in the expression of sigma factor genes during dehydration in terrestrial Nostoc HK-01 and aquatic Anabaena PCC 7120 were determined. The expression of the sigJ gene in terrestrial Nostoc HK-01, which is homologous to sigJ (alr0277) in aquatic Anabaena PCC 7120, was significantly induced in the mid-stage of dehydration. We constructed a higher-expressing transformant of the sigJ gene (HE0277) in Anabaena PCC 7120, and the transformant acquired desiccation tolerance. The results of Anabaena oligonucleotide microarray experiments showed that a comparatively large number of genes relating to polysaccharide biosynthesis were upregulated in the HE0277 cells. The extracellular polysaccharide released into the culture medium of the HE0277 cells was as much as 3.2-fold more than that released by the control cells. This strongly suggests that the group 3 sigma factor gene sigJ is fundamental and conducive to desiccation tolerance in these cyanobacteria.
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