Transcriptional and posttranscriptional components of psbA response to high light intensity in Synechococcus sp. strain PCC 7942

Abstract

The psbA genes, which encode the Dl protein of photosystem II, constitute a multigene family in the cyanobacterium Synechococcus sp. strain PCC 7942. Levels of messages from the three psbA genes change rapidly when cells are shifted from low-light to high-light conditions: the psbAI message level drops, whereas psbAII and psbAIII message levels increase dramatically. We examined the potential contributions of transcriptional and posttranscriptional processes in these high-light responses by subjecting cells that had been grown in a turbidostat at a standard light intensity (130 microeinsteins [1±EJ m-2 s-') to either the same or a higher light intensity (500 ,uE m-2 s-') in the presence or absence of rifampin. Northern (RNA blot) analysis of RNA isolated from cells subjected to high light showed that the increases in psbAII and psbAIII transcripts were blocked by rifampin. This suggests a transcriptional induction of these genes at high light intensities.Increased mRNA stability does not contribute to their accumulation in high-light conditions, since their half-life values did not increase relative to the half-lives measured at the standard light intensity. The rate of disappearance of the psbAI transcript in cells shifted to high light was diminished when either transcription or translation was blocked by rifampin or chloramphenicol, suggesting that accelerated degradation of the message requires de novo synthesis of a protein factor. When rifampin was added 10 min after the shift to high light intensity rather than before the shift, psbAl and psbAIII messages, but not the psbAII message, decayed at a faster rate. Susceptibility of the psbAIII transcript to the high-light-induced factor was also demonstrated by addition of chloramphenicol prior to the shift to high light. psbAIII transcript levels went up more than twofold higher in chloramphenicol-treated cells than in untreated cells, whereas psbAII transcript levels were unaffected by the inhibitor. These experiments provide evidence that either new or increased synthesis of a degradation factor which affects a subset of Synechococcus transcripts occurs in cells subjected to high light intensity.

mRNA stability is regulated by a coding‐region element and the unique 5′ untranslated leader sequences of the three Synechococcus psbA transcripts

“…The differential responses of the psbA genes to high light include accelerated degradation of the psbAI and psbAIII mRNAs, but not the psbAII mRNA, compared with their decay at standard light (Kulkarni et al, 1992). In order to identify the cis-acting elements that confer this post-transcriptional response, sequences from the psbAI minigene IC were replaced by corresponding sequences from psbAII or psbAIII.…”

“…We know that blocking the elongation of translating ribosomes by chloramphenicol increases the levels of the psbAI and psbAIII mRNAs at high light, whereas the level of psbAII mRNA is little affected (see Fig. 7, psbAI, psbAII, and psbAIII panels, later; Kulkarni et al, 1992). To test whether the short-lived IIA psbAII reporter message (Fig.…”

“…The differential regulation of these genes in response to high light intensity changes the ratio of D1 Form I:Form II and helps the cells better tolerate high light conditions (Kulkarni and Golden, 1995). The level of psbAI mRNA decreases at high light owing to a post-transcriptional accelerated decay of the message (Kulkarni et al, 1992). The psbAII and psbAIII messages are transcriptionally induced at high light; paradoxically, the decay of the psbAIII mRNA also increases.…”

“…Post-transcriptional regulation is an important layer of control in the response of a family of cyanobacterial photosystem II genes to changes in light intensity (Kulkarni et al, 1992;Golden, 1995). The three psbA genes of the cyanobacterium Synechococcus sp.…”

Two functionally distinct forms of the photosystem II reaction-center protein D1 in the cyanobacterium Synechococcus sp. PCC 7942.

“…Similar changes in the expression of the three psbA genes in Synechococcus sp. PCC 7942 also occur when cells grown in low light are shifted to higher photon irradiance (10,11).…”