Sequence of a cDNA encoding chicken high-mobility-group protein-2

The effect of the acidic tail on the DNA-binding properties of the HMG1,2 class of proteins: insights from tail switching and tail removal

The high-mobility group (HMG) proteins HMG1, HMG2 and HMG2a are relatively abundant vertebrate DNA-binding and bending proteins that bind with structure specificity, rather than sequence specificity, and appear to play an architectural role in the assembly of nucleoprotein complexes. They have two homologous "HMG-box" DNA-binding domains (which show about 80 % homology) connected by a short basic linker to an acidic carboxy-terminal tail that differs in length between HMG1 and 2. To gain insights into the role of the acidic tail, we examined the DNA-binding properties of HMG1, HMG2b and HMG2a from chicken erythrocytes (corresponding to HMG1, HMG2 and HMG2a in other vertebrates). HMG1, with the longest acidic tail, is less effective than HMG2a and 2b (at a given molar input ratio) in supercoiling relaxed, closed circular DNA, in inducing ligase-mediated circularisation of an 88 bp DNA fragment, and in binding to four-way DNA junctions in a gel-shift assay. Removal of the acidic tail increases the affinity of the HMG boxes for DNA and largely abolishes the differences between the three species. Switching the acidic tail of HMG1 for that of HMG2a or 2b gives hybrid proteins with essentially the same DNA-binding properties as HMG2a, 2b. The length (and possibly sequence) of the acidic tail thus appears to be the dominant factor in mediating the differences in properties between HMG1, 2a and 2b and finely tunes the rather similar DNA-binding properties of the tandem HMG boxes, presumably to fulfill different cellular roles. The tail is essential for structure-selective DNA-binding of the HMG boxes to DNA minicircles in the presence of equimolar linear DNA, and has little effect on the affinity for this already highly distorted DNA ligand, in contrast to binding to linear and four-way junction DNA.

Selection of a cDNA clone for chicken high-mobility-group 1 (HMG1) protein through its unusually conserved 3'-untranslated region, and improved expression of recombinant HMG1 in Escherichia coli

Screening of cDNA libraries for the homologous vertebrate proteins high mobility group (HMG) 1 and 2 using DNA probes based on the coding sequences is likely to result in isolation of both HMG1 and HMG2 clones, as well as pseudogenes, which may be transcribed at low levels. However, the 3'-untranslated regions (UTRs) of HMG1 and 2 are quite distinct, and unusually conserved across species. We have used this property to select the true chicken HMG1 cDNA clone from a chicken lymphocyte cDNA library in lambdagt11, using a probe based on the 3'-UTR of rat HMG1 cDNA. The chicken HMG1 cDNA clone is very similar to all the complete HMG1 cDNA clones isolated so far. We suggest that the sequence designated chicken HMG1 in the GenBank Data Library (Accession number D14314) is, in fact, that of HMG2a [and moreover that the recently reported mouse clone (Accession number AF022465), proposed to encode a new HMG protein, HMG4, is also likely to encode an HMG2a, based on the translated amino-acid sequence and 3'-UTR]. We also report much improved expression of intact recombinant HMG1 in Escherichia coli by the use of chloramphenicol rather than ampicillin selection and conditions that limit cell growth. This should be general for all members of the HMG1 (and 2) family which may be toxic to cells (possibly because of the long acidic tail), and may also prove useful in the production of other such proteins.

The cardiac-muscle specific enhancer-promoter of slow/cardiac troponin C binds HMG-2

The cardiac muscle-specific enhancer-promoter of the slow/cardiac troponin C (cTnC) gene contains five protein binding regions, four of which bind cardiac-myocyte specific proteins. We screened a stage 11 chick embryo expression library with a double-stranded oligonucleotide probe consisting of one of these regions, CEF-1. One of the clones obtained was the chicken high mobility group protein, HMG-2. An electrophoretic gel mobility shift assay (EMSA) showed a specific binding interaction between the HMG-2 protein and the dsDNA CEF-1 probe. The cardiac-specific enhancer region of cTnC contains at least one possible HMG binding region and it is in the CEF-1 sequence overlapping a known GATA-4 binding site. Mutation of the nucleotide sequence of this HMG binding region diminishes its protein binding ability and markedly decreases its cardiac specific transcriptional activity. HMG-2 is a DNA bending protein that is predominantly found in the nucleus in proliferating cells and in the cytoplasm of terminally differentiated cells. It is an integral and stabilizing factor in the transcription activation nucleoprotein complex and is often described as an 'architectural transcription factor'. It markedly stimulates the transcription of many genes, often in association with tissue-specific transcription factors. We believe that the presence of HMG-2 in the enhancer-promoter binding protein complex of cTnC augments DNA bending and facilitates the DNA binding and interaction of other tissue-specific factors (e.g. GATA-4, which also binds to this region). This would result in increased transcription of the cTnC gene during the proliferation phase of embryonic cardiac myocyte development.

RNA polymerase III transcription from the human U6 and adenovirus type 2 VAI promoters has different requirements for human BRF, a subunit of human TFIIIB

Mammalian TFIIIB can be separated into two fractions required for transcription of the adenovirus type 2 VAI gene, which have been designated 0.38M-TFIIIB and 0.48M-TFIIIB. While 0.48M-TFIIIB has not been characterized, 0.38M-TFIIIB corresponds to a TBP-containing complex. We describe here the purification of this complex, which consists of TBP and a closely associated polypeptide of 88 kDa, and the isolation of a cDNA corresponding to the 88-kDa polypeptide. The predicted protein sequence reveals that the 88-kDa polypeptide corresponds to a human homolog of the Saccharomyces cerevisiae BRF protein, a subunit of yeast TFIIIB. Human BRF (hBRF) probably corresponds to TFIIIB90, a protein previously cloned by Wang and Roeder (Proc. Natl. Acad. Sci. USA 92:7026-7030, 1995), although its predicted amino acid sequence differs from that reported for TFIIIB90 over a stretch of 67 amino acids as a result of frameshifts. Immunodepletion of more than 90 to 95% of the hBRF present in a transcription extract severely debilitates transcription from the tRNA-type VAI promoter but does not affect transcription from the TATA box-containing human U6 promoter, suggesting that the 0.38M-TFIIIB complex, and perhaps hBRF as well, is not required for U6 transcription.

Molecular cloning of polybromo, a nuclear protein containing multiple domains including five bromodomains, a truncated HMG-box, and two repeats of a novel domain

A number of transcription factors that act as adaptor proteins have been found to contain an 87 amino acid domain called the bromodomain. In a study to identify and characterise bromodomain proteins expressed in chicken cells, a novel gene has been isolated which encodes five repeats of the bromodomain. In addition, the encoded protein, termed polybromo, contains four other domains: an unusual truncated HMG box, two repeats of a novel domain which we term the BAH domain and a sequence related to a region within the regulatory domain of the DNA cytosine-5 methyltransferase enzyme. Polybromo was found to be related to a yeast protein U19102 which has two bromo domains, a BAH domain and the DNA methyltransferase-related sequence. Antibodies that were raised against polybromo were used in confocal microscopy analysis to show that the 180-kDa polybromo protein is located within the nucleus but excluded from the nucleolus. Gel filtration analysis of nuclear extracts demonstrate that polybromo is part of a large complex with a mass of approximately 2 million dalton.

cDNA sequence and structure of a trout HMG-2 gene. Evidence for a trout-specific 3'-untranslated region

A complete cDNA sequence (1026 bp) and a partial structure of a gene encoding the trout testis chromosomal HMG (high mobility group) 2 protein (HMG-T2) is presented. The deduced protein consists of 214 amino acids and shares over 80% similarity to the trout HMG-1 protein (HMG-T1) as well as to the mammalian or avian HMG-2 proteins. Northern blot analysis revealed two transcripts, a major one of 1.2 kb and a minor one of 1.6 kb. Southern analysis and polymerase chain reaction of trout genomic DNA indicated that the HMG-T2 gene is encoded by several introncontaining genes. The 5'-UTR (untranslated region) of the HMG-T2 is interrupted by an intron and the coding region of the HMG-T2 is divided into four exons by three relatively short introns (173, 91 and 78 bp). The exon/intron boundaries of trout HMG-2 are identical to those of human HMG-2, as reported earlier [Shirakawa and Yoshida, J. Biol. Chem. 267 (1992) 6641-6645], suggesting the evolution of the HMG-1/2 family genes from a common ancestor. Phylogenetic analysis indicated that the common ancestor of trout HMG-1/2 genes very likely diverged from the ancestor of the mammalian (or avian) HMG-1/2 genes before its separation into two distinct mammalian or (avian) HMG-1 and HMG-2 genes. Sequence comparisons of the 3'UTR of trout HMG-2 cDNA with the corresponding regions in the mammalian (or avian) HMG-2 revealed that the trout 3'-UTR was clearly distinct from the 3'-UTR of the mammalian or avian HMG-2 cDNAs which were otherwise remarkably well conserved.(ABSTRACT TRUNCATED AT 250 WORDS)