Involvement of a Nuclear-Encoded Basic Helix-Loop-Helix Protein in Transcription of the Light-Responsive Promoter ofpsbD  

Abstract

In the chloroplast psbD light-responsive promoter (LRP), a highly conserved sequence exists upstream from the bacterial Ϫ10/Ϫ35 elements. Multiple sequence-specific DNA binding proteins are predicted to bind to the conserved sequence as transcription factors. Using yeast one-hybrid screening of an Arabidopsis cDNA library, a possible DNA binding protein of the psbD LRP upstream sequence was identified. The protein, designated PTF1, is a novel protein of 355 amino acids (estimated molecular weight of 39.6) that contains a basic helix-loop-helix DNA binding motif in the predicted N-terminal region of the mature protein. Transient expression assay of PTF1-GFP fusion protein showed that PTF1 was localized in chloroplasts. Using the modified DNA sequence in the one-hybrid system, the ACC repeat was shown to be essential for PTF1 binding. The rate of psbD LRP mRNA accumulation was reduced in a T-DNA-inserted Arabidopsis ptf1 mutant. Compared with wild-type plants, the mutant had pale green cotyledons and its growth was inhibited under short-day conditions. These results suggest that PTF1 is a trans-acting factor of the psbD LRP.In the early stage of light-induced chloroplast development, the transcription activity of chloroplast genes increases, leading to increased mRNA accumulation (Klein and Mullet, 1990;Rapp et al., 1992; DuBell and Mullet, 1995). Transcription rates vary among promoters, and in most cases, they reflect the amount of transcripts and the stoichiometric composition of proteins (Rapp et al., 1992). Although mRNA stability is an important regulatory factor in mature chloroplasts (Kawaguchi et al., 1992; Kim et al., 1993;Staub and Maliga, 1993;Shiina et al., 1998), the transcription rate plays a primary role in controlling gene expression in developing chloroplasts (for review, see Mullet, 1993; Mayfield et al., 1995;Stern et al., 1997).Recent studies have clarified many of the molecular mechanisms of transcriptional regulation in plastids by successive cloning of RNA polymerase core and accessory subunits for plastid transcription (for review, see Maliga, 1998; Hess and Bö rner, 1999). One is nuclear-encoded bacteriophage-type RNA polymerase (Hedtke et al., 1997), which functions in transcription from the nuclear-encoded bacteriophage-type RNA polymerase promoters that exist in most nonphotosynthetic genes (Hajdukiewicz et al., 1997; Kapoor et al., 1997). On the other hand, many plastid genes have eubacterial 70 -type promoters, which are preceded by "Ϫ10" and "Ϫ35" elements (consensus TATAAT and TTGACA, respectively; Hanley- Bowdoin and Chua, 1987; Igloi and Kö ssel, 1992). These promoters are recognized by plastid-encoded RNA polymerase (PEP), which is composed of plastid-encoded catalytic core subunits associated with nuclear-encoded subunits (for review, see Link, 1996;Maliga, 1998; and Hess and Bö rner, 1999). Many plastid-factors have been cloned in higher plants (Isono et al., 1997;Tanaka et al., 1997; Kestermann et al., 1998;Tozawa et al., 1998). As in bacterial RNA polymerase, plastid-fac...

TheArabidopsis thalianaTCP transcription factors: A broadening horizon beyond development

“…Binding to the chloroplast psbD light-responsive promoter (LRP) and acting as a trans-acting factor of the psbD LRP. 127 Interacts with AHPs in Y2H, raising the possibility that TCP13 is regulated by CK signal. 48 …”

A comparative analysis of basic helix-loop-helix proteins, AtPTF1 and NtWIN4, with reference to plastid localization

“…AtPTF1 transcripts were predominantly found in leaves, but also detected at much low levels in stems, flowers and roots (Baba et al 2001), implying that AtPTF1 might be involved in photosynthesis. Indeed, GFP-fusion AtPTF1 was reported to be localized only in chloroplast of tobacco guard cells (Baba et al 2001).…”

“…PEND, a polypeptide containing a basic region plus a leucine-zipper region, is present in the inner envelope membrane of developing chloroplasts and specifically binds to TAAGAAGT (Sato et al 1998;Sato and Ohta 2001). Another example is Arabidopsis PTF1 (AtPTF1), which is a nuclear-encoded basic helix-loop-helix (bHLH) protein reported to function as a transcription factor in plastids (Baba et al 2001).…”

“…AtPTF1 was first isolated by yeast one-hybrid screening using a modified psbD light-responsive promoter (LRP and found to bind to the ACC repeat region of the LRP sequence, localized in chloroplasts) (Baba et al 2001). There is evidence that it regulates LRP transcription in plastids, and involvement in transcription of the psbD LRP gene has been suggested (Baba et al 2001). Recently, we identified another bHLH protein that is translocated into plastids and participates in hypersensitive cell death in tobacco plants (Kodama and Sano 2006).…”

“…However, as a rare and exceptional case, cellular localization other than in the nucleus has been reported for AtPTF1, in plastids. AtPTF1 was first isolated by yeast one-hybrid screening using a modified psbD light-responsive promoter (LRP and found to bind to the ACC repeat region of the LRP sequence, localized in chloroplasts) (Baba et al 2001). There is evidence that it regulates LRP transcription in plastids, and involvement in transcription of the psbD LRP gene has been suggested (Baba et al 2001).…”