The 5'-upstream Cis-Acting Sequences of a Cyanobacterial psbA Gene: Analysis of Their Roles in Basal, Light-Dependent and Circadian Transcription

Mol Genet Genomics. 2002 Jul;267(5):684-94. doi: 10.1007/s00438-002-0704-3. Epub 2002 Jun 26.


Transcription of the psbA2 gene in the unicellular photosynthetic cyanobacterium Microcystis aeruginosa K-81 is modulated by light and follows a circadian rhythm. In this study, we further characterized psbA transcription using a series of 5'-upstream deletions and mutant promoters which were tested in both photosynthetic and non-photosynthetic bacteria. Specific psbA2 transcripts were obtained from a minimal promoter sequence (-38/+14) with Escherichia coli RNA polymerases (RNAPs) both in vivo and in vitro, indicating the presence of a common regulatory mechanism for basal transcription. A DNase I footprinting assay showed that the E. coli RNAP, which is structurally similar to that of cyanobacteria, specifically binds to a large segment (from -115 to +23) of the sequence upstream of psbA2. In cyanobacteria, the -10 sequence (TAGTAT), but not the -35 motif (TTTACA), is essential for basal transcription by homologous and heterologous RNAPs that contain the major sigma factor. Each of the conserved thymidine nucleotides at positions -12 and -7 (underlined above) was essential, and both an insertion and a deletion in the spacer region of the promoter caused reductions in transcription. RNAP was able to bind to a mutant promoter lacking the -10 sequence, though this did not actually lead to transcription. Interestingly, a high level of arrhythmic circadian transcription was observed in mutants lacking the -35 region. In contrast, a mutation in the AU-box mutation, which controls the stability of the psbA2 mRNA, did not affect the circadian pattern of transcription. These findings demonstrate that light-dependent psbA2 expression is controlled at the transcriptional and post-transcriptional levels, whereas the circadian pattern of expression is regulated at the transcriptional level.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Sequence
  • Circadian Rhythm / genetics
  • Cyanobacteria / genetics*
  • Escherichia coli
  • Gene Expression Regulation, Bacterial*
  • Light
  • Molecular Sequence Data
  • Photosynthetic Reaction Center Complex Proteins / genetics*
  • Photosystem II Protein Complex*
  • Promoter Regions, Genetic*
  • Sequence Deletion
  • Untranslated Regions
  • Up-Regulation


  • Photosynthetic Reaction Center Complex Proteins
  • Photosystem II Protein Complex
  • Untranslated Regions
  • photosystem II, psbA subunit