Background: Plastid-encoded eubacterial-type RNA polymerase (PEP) plays a critical
role in the transcription of photosynthesis genes in chloroplasts. Notably, some of the reaction
center genes, including psaA, psaB, psbA, and psbD genes, are differentially transcribed by PEP in
mature chloroplasts. However, the molecular mechanism of promoter selection in the reaction
center gene transcription by PEP is not well understood.
Objective: Sigma factor proteins direct promoter selection by a core PEP in chloroplasts as well as
bacteria. AtSIG5 is a unique chloroplast sigma factor essential for psbD light-responsive promoter
(psbD LRP) activity. To analyze the role of AtSIG5 in chloroplast transcription in more detail, we
assessed the effect of AtSIG5 hyper-expression on the transcription of plastid-encoded genes in
chloroplast transgenic plants.
Results: The chloroplast transgenic tobacco (CpOX-AtSIG5) accumulates AtSIG5 protein at
extremely high levels in chloroplasts. Due to the extremely high-level expression of recombinant
AtSIG5, most PEP holoenzymes are most likely to include the recombinant AtSIG5 in the CpOXAtSIG5
chloroplasts. Thus, we can assess the promoter preference of AtSIG5 in vivo. The
overexpression of AtSIG5 significantly increased the expression of psbD LRP transcripts encoding
PSII reaction center D2 protein and psaA/B operon transcripts encoding PSI core proteins.
Furthermore, run-on transcription analyses revealed that AtSIG5 preferentially recognizes the
psaA/B promoter, as well as the psbD LRP. Moreover, we found that psbD LRP is constitutively
active in CpOX-AtSIG5 plants irrespective of light and dark.
Conclusion: AtSIG5 probably plays a significant role in differential transcription of reaction center
genes in mature chloroplasts.