Transcriptional and posttranscriptional control of mRNA from IrtA, a light-repressed transcript in Synechococcus sp. PCC 7002

Abstract

Transcription regulation and transcript stability of a light-repressed transcript, IrtA, from the cyanobacterium Synechococcus sp. PCC 7002 were studied using ribonuclease protection assays. The transcript for IrtA was not detected in continuously illuminated cells, yet transcript levels increased when cells were placed in the dark. A lag of 20 to 30 min was seen in the accumulation of this transcript after the cells were placed in the dark. Transcript synthesis continued in the dark for 3 h and the transcript levels remained elevated for at least 7 h. The addition of 10 μM rifampicin to illuminated cells before dark adaptation inhibited the transcription of IrtA in the dark. Upon the addition of rifampicin to 3-h dark-adapted cells, IrtA transcript levels remained constant for 30 min and persisted for 3 h. A 3-h half-life was estimated in the dark, whereas a 4-min half-life was observed in the light. Extensive secondary structure was predicted for this transcript within the 5′ untranslated region, which is also present in the 5′ untranslated region of IrtA from a different cyanobacterium, Synechocystis sp. PCC 6803. Evidence suggests that IrtA transcript stability is not the result of differences in ribonuclease activity from dark to light. Small amounts of IrtA transcript were detected in illuminated cells upon the addition of 25 μg ml-1 chloramphenicol. The addition of chloramphenicol to dark-adapted cells before illumination allowed detection of the IrtA transcript for longer times in the light relative to controls without chloramphenicol. These results suggest that IrtA mRNA processing in the light is different from that in the dark and that protein synthesis is required for light repression of the IrtA transcript.