Specificity versus redundancy in the RAP2.4 transcription factor family of
Arabidopsis thaliana: transcriptional regulation of genes for chloroplast
peroxidases
Background The Arabidopsis ERFIb / RAP2.4 transcription factor family consists
of eight members with highly conserved DNA binding domains. Selected members
have been characterized individually, but a systematic comparison is pending.
The redox-sensitive transcription factor RAP2.4a mediates chloroplast-to-
nucleus redox signaling and controls induction of the three most prominent
chloroplast peroxidases, namely 2-Cys peroxiredoxin A (2CPA) and thylakoid-
and stromal ascorbate peroxidase (tAPx and sAPx). To test the specificity and
redundancy of RAP2.4 transcription factors in the regulation of genes for
chloroplast peroxidases, we compared the DNA-binding sites of the
transcription factors in tertiary structure models, analyzed transcription
factor and target gene regulation by qRT-PCR in RAP2.4, 2-Cys peroxiredoxin
and ascorbate peroxidase T-DNA insertion lines and RAP2.4 overexpressing lines
of Arabidopsis thaliana and performed promoter binding studies. Results All
RAP2.4 proteins bound the tAPx promoter, but only the four RAP2.4 proteins
with identical DNA contact sites, namely RAP2.4a, RAP2.4b, RAP2.4d and
RAP2.4h, interacted stably with the redox-sensitive part of the 2CPA promoter.
Gene expression analysis in RAP2.4 knockout lines revealed that RAP2.4a is the
only one supporting 2CPA and chloroplast APx expression. Rap2.4h binds to the
same promoter region as Rap2.4a and antagonizes 2CPA expression. Like the
other six RAP2.4 proteins, Rap2.4 h promotes APx mRNA accumulation.
Chloroplast ROS signals induced RAP2.4b and RAP2.4d expression, but these two
transcription factor genes are (in contrast to RAP2.4a) insensitive to low 2CP
availability, and their expression decreased in APx knockout lines. RAP2.4e
and RAP2.4f gradually responded to chloroplast APx availability and activated
specifically APx expression. These transcription factors bound, like RAP2.4c
and RAP2.4g, the tAPx promoter, but hardly the 2CPA promoter. Conclusions The
RAP2.4 transcription factors form an environmentally and developmentally
regulated transcription factor network, in which the various members affect
the expression intensity of the others. Within the transcription factor
family, RAP2.4a has a unique function as a general transcriptional activator
of chloroplast peroxidase activity. The other RAP2.4 proteins mediate the
fine-control and adjust the relative availability of 2CPA, sAPx and tAPx