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Keywords:

  • plastid sigma factor;
  • cryptochrome;
  • chloroplast;
  • transcription;
  • plastid-encoded RNA polymerase-dependent promoter;
  • Arabidopsis

Summary

In higher plants, multiple nuclear-encoded sigma factors activate select subsets of plastid gene promoters in a partially redundant manner. We analysed the light induction profiles of transcripts from six Arabidopsis sigma factor (AtSIG) genes in mature leaves, focusing on the effects of wavelength and intensity. Red-light illumination (660 nm) of dark-adapted plants strongly induced AtSIG1 transcripts, while blue-light illumination (470 nm) caused strong and rapid induction of AtSIG1 and AtSIG5 transcripts. The fluence response differed in blue-light-responsive rapid induction in AtSIG1 and AtSIG5. AtSIG1 transcripts increased to plateau with a threshold of 2 μmol m−2 sec−1 under all fluences examined (1–50 μmol m−2 sec−1), and AtSIG5 transcripts were induced with a distinct two-phase profile, with the lower-fluence induction similar to that of AtSIG1 and further enhancement with increasing fluences greater than 10 μmol m−2 sec−1. Blue-light-receptor mutational analysis revealed that AtSIG5-specific two-phase induction is mediated through cryptochrome 1 and cryptochrome 2 at lower fluences and more significantly through cryptochrome 1 at higher fluences. In mature chloroplasts, the promoters of psbA and psbD are predominantly recognized by AtSIG5 among six sigma factors. Using a protoplast transient expression assay with AtSIG5AtSIG1 chimeric genes, we present evidence that AtSIG5 contains determinants for activating the psbD blue-light-responsive promoter (BLRP) in region 4.2 rather than region 2.4. Amino acid scanning within AtSIG5 region 4.2 revealed that Asn484, but not Arg493, functions as a key residue for psbD BLRP activation. Arginine 493 may be involved in psbA promoter recognition.