Vipp1 is essential for the biogenesis of Photosystem I but not thylakoid membranes in Synechococcus sp. PCC 7002

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 PcpcBA 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 PpsaAB 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.