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Na + /H + antiporters mediated pH regulation is one of the known mechanism(s), which advocates a possible role of the antiporters in the alkaline pH tolerance of Arthrospira platensis NIES-39 too.Seven putative Na + /H + antiporters have been reported in A. platensis. We have characterized these seven antiporters, where A1, Q2, L2 and L6 belong to the CPA1 family whereas C5, D5 and O6 belong to CPA2 family through various In-silico analysis. Conserved domain analysis of these seven putative antiporters indicate the presence of nine different kinds of domains. Out of these nine domains, six domains function as monovalent cation-proton antiporters and two as the universal stress protein (Usp) category. Transcription profile of these seven antiporters was also generated at three different pH and time frames which showed a significant difference in the m-RNA levels at pH 7, 9 and at 11 along with a temporal pattern of the expression profile. The Insilico and the real time PCR analysis, put together, suggest the active participation of these seven putative Na + /H + antiporters in alkaline pH homeostasis of this cyanobacterial strain where CPA1 subfamily antiporters play a major role.

Addition of nitrate (NO3 (-)) to a suspension of NO3 (-)-depleted Spirulina platensis PCC 7345 restored back the PSII reaction centre efficiency. Using RAPD/RT-PCR differential display, we have detected the upregulation of a novel gene in filaments starved of nitrogen when enriched with NO3 (-). Semiquantitative RT-PCR clearly shows that NO3 (-) repletion induces the expression of this gene. Its nucleotide sequence is homologous to multi sensor histidine kinase (hstk) from Spirulina platensis NIES-39 and Spirulina maxima CS-328 genes. The open reading frame of this gene encodes a protein of 98 amino acids contains sensory domain of PAS domain.

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