Abstract
The biogenesis of thylakoid membranes in cyanobacteria is presently not well understood, but the vipp1 gene product has been suggested to play an important role in this process. Previous studies in Synechocystis sp. PCC 6803 reported that vipp1 (sll0617) was essential. By constructing a fully segregated null mutant in vipp1 (SynPCC7002_A0294) in Synechococcus sp. PCC 7002, we show that Vipp1 is not essential. Spectroscopic studies revealed that Photosystem I (PS I) was below detection limits in the vipp1 mutant, but Photosystem II (PS II) was still assembled and was active. Thylakoid membranes were still observed in vipp1 mutant cells and resembled those in a psaAB mutant that completely lacks PS I. When the vipp1 mutation was complemented with the orthologous vipp1 gene from Synechocystis sp. PCC 6803 that was expressed from the strong P(cpcBA) promoter, PS I content and activities were restored to normal levels, and cells again produced thylakoids that were indistinguishable from those of wild type. Transcription profiling showed that psaAB transcripts were lower in abundance in the vipp1 mutant. However, when the yfp gene was expressed from the P(psaAB) promoter in the presence and the absence of Vipp1, no difference in YFP expression was observed, which shows that Vipp1 is not a transcription factor for the psaAB genes. This study shows that thylakoids are still produced in the absence of Vipp1 and that normal thylakoid biogenesis in Synechococcus sp. PCC 7002 requires expression and biogenesis of PS I, which in turn requires Vipp1.
Highlights
Vipp1 was previously thought to be essential for viability and biogenesis of thylakoid membranes
Some observations made with vipp1 merodiploid strains suggested that these merodiploid strains had greatly reduced levels of Photosystem I (PS I), and a strain in which the psaAB genes had been deleted was used for inactivation of the vipp1 gene
Thylakoid Membranes in a psaAB Deletion Mutant— previous studies had suggested that Vipp1 was required for thylakoid membrane biogenesis, the results shown above suggested that Vipp1 is required for psaAB expression or the biogenesis of PS I
Summary
Vipp was previously thought to be essential for viability and biogenesis of thylakoid membranes. PCC 6803, but the level of Vipp could be lowered, in none of these studies could null mutations of the vipp gene be fully segregated. PCC 6803 expressed from the strong PcpcBA promoter, the resulting strain regained the ability to grow photoautotrophically and regained all other phenotypic properties of the wild type Characterization of these strains showed that Vipp is required for biogenesis of PS I and that PS I is required for the biogenesis of “normal” thylakoid membranes in Synechococcus sp.
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