D1 polypeptide degradation may regulate psbA gene expression at transcriptional and translational levels in Synechocystis sp. PCC 6803

Abstract

Light has been suggested to regulate both synthesis and degradation of the Photosystem II (PS II) reaction centre polypeptide D1, encoded by the psbA gene. The modified degradation rate of the D1 polypeptide in site-directed Synechocystis sp PCC 6803 D1 mutants CA1 [del(E242-E244);Q241H], E243K and E229D has provided a tool to determine whether the rate of D1 polypeptide synthesis is directly regulated by light-intensity-related factors or by a control mechanism mediated by light-dependent degradation of the D1 polypeptide. In vivo accumulation of [(35)S] methionine into the D1 polypeptide was found to correlate with D1 polypeptide degradation rather than with incident irradiance. This suggests that the degradation rate of the D1 polypeptide regulates its own synthesis at translational level. Furthermore, several fold differences in the psbA mRNA levels were measured between D1 mutant strains, indicating that the psbA gene transcription is not solely under light control.

Exposure of Synechocystis 6803 cells to series of single turnover flashes increases the psbA transcript level by activating transcription and down‐regulating psbA mRNA degradation

“…Activation of the psbA gene transcription upon shift of Synechocystis cells from low to high light conditions is a well documented phenomenon [6,10]. This phenomenon was suggested to be related to the light-induced degradation of the D1 protein [10].…”

“…RNA extraction was performed as described by Tyystja ërvi et al [10]. For DNA probes, the whole coding regions of the psbA2, psbD2, petA genes and the N-terminal half of psaA gene were ampli¢ed by PCR from genomic DNA of Synechocystis 6803.…”

“…Prehybridization, hybridization and washing after hybridization were performed as described earlier [10]. To measure the amount of transcripts, the X-ray ¢lms were scanned with a laser densitometer (Ultroscan XL, LKB) and the area of the peak was measured.…”

“…Transcription initiation has been proposed to be the main regulatory level of the psbA gene expression in Synechocystis 6803 [6], and a direct correlation between the transcription activity and light intensity has been proposed [6,7]. However, later studies have shown that the psbA gene expression in Synechocystis is not only regulated at the level of transcription, but also at the level of message stability [8,9] and translation [10]. Furthermore, studies with mutant strains have shown that the transcription rate of the psbA gene is not directly dependent on light [10,11].…”

“…However, later studies have shown that the psbA gene expression in Synechocystis is not only regulated at the level of transcription, but also at the level of message stability [8,9] and translation [10]. Furthermore, studies with mutant strains have shown that the transcription rate of the psbA gene is not directly dependent on light [10,11]. The regulation of psbA message level is of particular interest since psbA gene transcripts and their translation products are needed for the PSII repair cycle that maintains the PSII activity under varying light conditions [12,13].…”