Two genes for the high mobility group protein HMG-Y are present in the genome of Canavalia gladiata D.C

Potential function of CbuSPL and gene encoding its interacting protein during flowering in Catalpa bungei

BACKGROUND

"Bairihua", a variety of the Catalpa bungei, has a large amount of flowers and a long flowering period which make it an excellent material for flowering researches in trees. SPL is one of the hub genes that regulate both flowering transition and development.

RESULTS

SPL homologues CbuSPL9 was cloned using degenerate primers with RACE. Expression studies during flowering transition in "Bairihua" and ectopic expression in Arabidopsis showed that CbuSPL9 was functional similarly with its Arabidopsis homologues. In the next step, we used Y2H to identify the proteins that could interact with CbuSPL9. HMGA, an architectural transcriptional factor, was identified and cloned for further research. BiFC and BLI showed that CbuSPL9 could form a heterodimer with CbuHMGA in the nucleus. The expression analysis showed that CbuHMGA had a similar expression trend to that of CbuSPL9 during flowering in "Bairihua". Intriguingly, ectopic expression of CbuHMGA in Arabidopsis would lead to aberrant flowers, but did not effect flowering time.

CONCLUSIONS

Our results implied a novel pathway that CbuSPL9 regulated flowering development, but not flowering transition, with the participation of CbuHMGA. Further investments need to be done to verify the details of this pathway.

Physcomitrella HMGA-type proteins display structural differences compared to their higher plant counterparts

High mobility group (HMG) proteins of the HMGA family are chromatin-associated proteins that act as architectural factors in nucleoprotein structures involved in gene transcription. To date, HMGA-type proteins have been studied in various higher plant species, but not in lower plants. We have identified two HMGA-type proteins, HMGA1 and HMGA2, encoded in the genome of the moss model Physcomitrella patens. Compared to higher plant HMGA proteins, the two Physcomitrella proteins display some structural differences. Thus, the moss HMGA proteins have six (rather than four) AT-hook DNA-binding motifs and their N-terminal domain lacks similarity to linker histone H1. HMGA2 is expressed in moss protonema and it localises to the cell nucleus. Typical of HMGA proteins, HMGA2 interacts preferentially with A/T-rich DNA, when compared with G/C-rich DNA. In cotransformation assays in Physcomitrella protoplasts, HMGA2 stimulated reporter gene expression. In summary, our data show that functional HMGA-type proteins occur in Physcomitrella.

Overlapping expression patterns among the genes encodingArabidopsischromosomal high mobility group (HMG) proteins

High mobility group (HMG) proteins are usually considered ubiquitous components of the eukaryotic chromatin. Using HMG gene promoter-GUS reporter gene fusions we have examined the expression of the reporter gene in transgenic Arabidopsis plants. These experiments have revealed that the different HMGA and HMGB promoters display overlapping patterns of activity, but they also show tissue- and developmental stage-specific differences. Moreover, leader introns that are present in some of the HMGB genes can modulate reporter gene expression. The differential HMG gene expression supports the view that the various HMG proteins serve partially different architectural functions in plant chromatin.

Interaction of wheat high-mobility-group proteins with four-way-junction DNA and characterization of the structure and expression of HMGA gene

Plant high-mobility-group (HMG) chromosomal proteins are the most abundant and ubiquitous nonhistone proteins found in the nuclei of higher eukaryotes. There are only two families of HMG proteins, namely, HMGA and HMGB in plants. The cDNA encoding wheat HMGa protein was isolated and characterized. Wheat HMGA cDNA encodes a protein of 189 amino acid residues. At its N terminus, there is a histone H1-like structure, which is a common feature of plant HMGA proteins, followed by four AT-hook motifs. Polymerase chain reaction results show that the gene contains a single intron of 134 bp. All four AT-hook motifs are encoded by the second exon. Northern blot results show that the expression of HMGA gene is much higher in organs undergoing active cell proliferation. Gel retardation analysis show that wheat HMGa, b, c and histone H1 bind to four-way-junction DNA with high binding affinity, but affinity is dramatically reduced with increasing Mg(2+) and Na(+) ion concentration. Competition binding studies show that proteins share overlapping binding sites on four-way-junction DNA. HMGd does not bind to four-way-junction DNA.

HMGI/Y proteins: flexible regulators of transcription and chromatin structure

The mammalian HMGI/Y (HMGA) non-histone proteins participate in a wide variety of cellular processes including regulation of inducible gene transcription, integration of retroviruses into chromosomes and the induction of neoplastic transformation and promotion of metastatic progression of cancer cells. Recent advances have contributed greatly to our understanding of how the HMGI/Y proteins participate in the molecular mechanisms underlying these biological events. All members of the HMGI/Y family of 'high mobility group' proteins are characterized by the presence of multiple copies of a conserved DNA-binding peptide motif called the 'AT hook' that preferentially binds to the narrow minor groove of stretches of AT-rich sequence. The mammalian HMGI/Y proteins have little, if any, secondary structure in solution but assume distinct conformations when bound to substrates such as DNA or other proteins. Their intrinsic flexibility allows the HMGI/Y proteins to participate in specific protein-DNA and protein-protein interactions that induce both structural changes in chromatin substrates and the formation of stereospecific complexes called 'enhanceosomes' on the promoter/enhancer regions of genes whose transcription they regulate. The formation of such regulatory complexes is characterized by reciprocal inductions of conformational changes in both the HMGI/Y proteins themselves and in their interacting substrates. It may well be that the inherent flexibility of the HMGI/Y proteins, combined with their ability to undergo reversible disordered-to-ordered structural transitions, has been a significant factor in the evolutionary selection of these proteins for their functional role(s) in cells.

Isolation of a Brassica napus L. cDNA encoding a putative high-mobility-group HMG I/Y protein

A cDNA encoding a high-mobility-group protein has been isolated from a microspore-specific library of Brassica napus. The 930 bp cDNA contains a 612 bp open reading frame encoding a protein of 203 amino acids residues exhibiting significant homology to HMG-I/Y protein from Arabidopsis thaliana (62%). The predicted protein contains four copies of the 'AT-hook' motif which is involved in binding A/T-rich DNA. Southern blotting indicates that the HMG-I/Y gene is a single-copy gene in B. napus. Transcription of the HMG-I/Y gene was detected in all tissues examined, with the highest expression in pollen-derived embryos. In situ localization studies of flower organs indicate the transcript to be preferentially located in petals and sepals. Subcellular localization analysis performed during pollen development showed that the transcript of the HMG-I/Y gene is predominantly associated with polysomes.

Transcription factors and their genes in higher plants functional domains, evolution and regulation

A typical plant transcription factor contains, with few exceptions, a DNA-binding region, an oligomerization site, a transcription-regulation domain, and a nuclear localization signal. Most transcription factors exhibit only one type of DNA-binding and oligomerization domain, occasionally in multiple copies, but some contain two distinct types. DNA-binding regions are normally adjacent to or overlap with oligomerization sites, and their combined tertiary structure determines critical aspects of transcription factor activity. Pairs of nuclear localization signals exist in several transcription factors, and basic amino acid residues play essential roles in their function, a property also true for DNA-binding domains. Multigene families encode transcription factors, with members either dispersed in the genome or clustered on the same chromosome. Distribution and sequence analyses suggest that transcription factor families evolved via gene duplication, exon capture, translocation, and mutation. The expression of transcription factor genes in plants is regulated at transcriptional and post-transcriptional levels, while the activity of their protein products is modulated post-translationally. The purpose of this review is to describe the domain structure of plant transcription factors, and to relate this information to processes that control the synthesis and action of these proteins.

The HMG-I/Y protein PF1 stimulates binding of the transcriptional activator GT-2 to the PHYA gene promoter

The DNA-binding proteins PF1 and GT-2 are factors that bind to different functionally defined, positively acting cis-elements in the PHYA genes of oat and rice, respectively. PF1 is an HMG-I/Y protein, with its cognate cis-element being an AT-rich sequence, designated PE1, whereas GT-2 is a transcriptional activator with twin DNA binding domains that recognize a triplet of GT-boxes in a complex motif designated GTE. To further define the DNA-binding activity of PF1 and to explore potential inter-relationships between the two factors, we have performed a series of in vitro DNA-binding experiments with both PE1 and GTE target sites. The data show that, consistent with its membership of the HMG-I/Y protein family, PF1 can bend DNA when bound to PE1. In addition, PF1 can bind promiscuously, with varying affinity, to other AT-containing motifs, including GTE. When co-incubated with GT-2, PF1 enhances the specific DNA-binding activity of GT-2 toward GTE, the first report of such activity for a plant HMG-I/Y protein. This enhancement takes place without demonstrable physical contact between the two proteins, suggesting the possibility of a novel, indirect mechanism of recruitment involving DNA target-site pre-conditioning. The evidence indicates therefore that PF1 and GT-2 do not perform functionally equivalent roles in positively regulating oat and rice PHYA gene expression. However, the data suggest the possibility that PF1 may act as an architectural factor, promiscuously recognizing a spectrum of AT-containing elements in plant promoters, with the general function of catalyzing enhanced binding of conventional cognate transcriptional regulators to these elements via DNA bending.

Plant chromosomal HMGI/Y proteins and histone H1 exhibit a protein domain of common origin

The chromosomal high-mobility-group (HMG) proteins of the HMGI/Y family interact with A/T-rich stretches in duplex DNA, and are considered assistant factors in transcriptional regulation. A cDNA encoding an HMGI/Y protein of 190 amino acid residues was isolated from maize and characterized. Like other plant HMGI/Y proteins, the maize HMGI/Y protein contains four copies of the AT-hook DNA-binding motif and an amino-terminal 'histone H1-like region' with a similarity to the globular domain of H1. The maize hmgi/y gene that was isolated from a genomic DNA library contains a single intron that is localized in the region of sequence similarity to histone H1. Interestingly, the genes encoding plant H1 contain an intron at exactly the same relative position, indicating an evolutionary relationship of the plant genes encoding HMGI/Y and H1 proteins.

Isolation and characterization of a cDNA encoding a high mobility group protein HMG-1 from Canavalia gladiata D.C

A cDNA clone encoding a HMG-1 protein from maturing seeds of Canavalia gladiata was isolated and characterized with respect to its sequence, genomic organization and the expression pattern in seeds. The predicted polypeptide had the characteristic conserved motifs of the HMG-1/2 protein including N-terminal basic region, one HMG-box and polyacidic carboxy terminus. Southern blot analysis suggested that the HMG-1 gene is a single copy gene. Northern blot analysis indicated that the HMG-1 gene was expressed both in maturing and germinated seeds.