ABSTRACT
Throughout the history of oxygen evolution, two types of photosystem reaction centres (PSI and PSII) have worked in a coordinated manner. The oxygen evolving centre is an integral part of PSII, and extracts an electron from water. PSI accepts the electron, and accumulates reducing power. Traditionally, PSI and PSII are thought to be spatially dispersed. Here, we show that about half of PSIIs are physically connected to PSIs in Arabidopsis thaliana. In the PSI-PSII complex, excitation energy is transferred efficiently between the two closely interacting reaction centres. PSII diverts excitation energy to PSI when PSII becomes closed-state in the PSI-PSII complex. The formation of PSI-PSII complexes is regulated by light conditions. Quenching of excess energy by PSI might be one of the physiological functions of PSI-PSII complexes.
Subject(s)
Arabidopsis Proteins/physiology , Arabidopsis , Energy Transfer , Oxygen/metabolism , Photosystem I Protein Complex/physiology , Photosystem II Protein Complex/physiology , Electron Transport , LightABSTRACT
Transcription factor Sp1 has three tandem repeats of a Cys2His2-type zinc finger motif and specifically binds to GC box DNA. Although the DNA binding mode of the three zinc fingers of Sp1 is predicted to be similar to that of Zif268, this model does not explain the DNA binding property of the N-terminal zinc finger (finger 1). To understand the DNA recognition mode of Sp1, we have performed detailed analyses for the contribution of finger 1 to the high-affinity binding to the GC box DNA and for the interaction mechanism between finger 1 and DNA. Results of electrophoretic analyses using finger-deleted mutants of Sp1 and GC box mutants in the finger-contacting subsite demonstrate that the contribution of finger 1 to the total DNA binding affinity is lower than that of the C-terminal finger 3 but is dispensable for the high-affinity binding. The DNA sequence selectivity of finger 1 at the 3'-portion of the GC box is lower than that of fingers 2 and 3 at the 5'-portion. Alanine scanning mutagenesis in the alpha-helix of finger 1 reveals that Lys-1 immediately preceding the helix is important for the recognition of the two guanine bases, but other putative key amino acids do not affect the DNA binding. These results demonstrate that (1) the contribution of finger 1 to the DNA binding affinity and the sequence selectivity of Sp1 is smaller than that of fingers 2 and 3 and (2) the interaction mechanism between finger 1 and DNA is different from the Zif268 model. DNA interaction of Sp1 finger 1 has also been discussed in connection with that of TFIIIA or WT1.
Subject(s)
DNA/metabolism , Sp1 Transcription Factor/metabolism , Zinc Fingers/genetics , Amino Acid Sequence , Amino Acid Substitution/genetics , Amino Acids/genetics , Amino Acids/metabolism , Consensus Sequence/genetics , Humans , Molecular Sequence Data , Mutagenesis, Site-Directed , Protein Binding/genetics , Sequence Deletion , Sp1 Transcription Factor/geneticsABSTRACT
To clarify binding properties of the first zinc finger of Sp1, finger 1, to GC-box DNA, two-finger mutant peptides Sp1 (zf12) and Sp1 (zf23) were created and their DNA binding characteristics have been compared with those of native three-zinc finger protein Sp1. Some gel electrophoretic experiments involving DNase I footprinting and gel mobility shift assays reveal that finger 1 is not essential to sequence-specific DNA recognition, but moderately contributes to total binding affinity and overall sequence specificity by three zinc fingers of Sp1.
