Organization of the human myoglobin gene.

Cross-hybridization of the grey seal myoglobin gene to human DNA detected a single human myoglobin gene plus an extensive family of sequences apparently related to the central exon of this gene. The functional human gene is 10.4 kb long and has a haemoglobin-like three exon/two intron structure with long non-coding regions similar to its seal homologue. At least 300 bp of 5'-flanking region are closely homologous between the two genes, with the exception of a divergent purine-rich region 68-114 bp upstream of the cap site. A diverged tandem repetitive sequence based on (GGAT)165 is located 1100-1750 bp upstream from the gene; internal homology units within this sequence suggest sequence homogenization by gene microconversions. A second 33-bp tandem repeat element in the first intron is flanked by a 9-bp direct repeat, shares homology with other tandem repetitive elements in the human genome and may represent a novel form of transposable element.     

The human alpha-fetoprotein gene. Sequence organization and the 5' flanking region

The human alpha-fetoprotein (AFP) gene was isolated into three overlapping clones in bacteriophage lambda vectors and its sequence organization analyzed by restriction endonuclease mapping and nucleotide sequencing. The human AFP gene is about 20 kilobase pairs long and contains 15 exons and 14 introns. The overall organization of the human AFP gene is similar to that of the mouse AFP gene, with all but two exons showing identical sizes. Nucleotide sequences at all exon/intron junctions display similarity to the consensus boundary sequence (Breathnach, R., and Chambon, P. (1981) Annu. Rev. Biochem. 50, 349-383), with the GT-AG rule applied to the splicing point. The cap site maps 44 nucleotides upstream from the translation initiation site. The "TATA box" is located 27 nucleotides upstream from the putative cap site and is flanked by sequences with dyad symmetry. The TATA box can thus be placed in the loop portion of a possible stem-loop structure formed by intrastrand base-pairing. Other characteristic nucleotide sequences in the 5' flanking region include a CCAAC pentamer, a 14-base pair (bp) enhancer-like sequence, and a 9-bp sequence homologous to the glucocorticoid responsive element. A long (90 bp) direct repeat and several alternating purine/pyrimidine sequences are also present in the 5' flanking region. A 736-bp sequence of the 5' flanking region adjacent to the cap site of the human AFP gene shows a 61% similarity with the corresponding region of the mouse AFP gene. There are two Alu family sequences and two poly(dT-dG) repeats in the human AFP gene that show different distribution patterns from those in the mouse AFP gene.     

Repetitive DNA (TGGA)n 5' to the human myelin basic protein gene: a new form of oligonucleotide repetitive sequence showing length polymorphism.

DNA 5' to the human myelin basic protein (MBP) gene, mapped to 18q22----qter, is known to manifest multiallelic DNA length variation with heterozygosity of at least 45%. Isolation of genomic DNA containing the MBP gene first exon and its 5' flanking region reveals that this polymorphism arises from a 994-bp region of the diverged tandem repeat (TGGA)249. This sequence is located from 1082 to 2075 bp upstream of the MBP initiator methionine. The repetitive sequence is 18% diverged from (TGGA)249 and from analysis of higher order subsequence reiterations appears to have undergone extensive recombination. The pattern of higher order repetition suggests that multiple crossover and gene conversion events have occurred within a 1.0-kb region. Molecular clones of this sequence represent essentially the longest allelic form of this region seen in Southern transfer analysis. This repetitive DNA is similar to a sequence 5' to the human myoglobin gene.     

Positive and negative regulatory DNA elements including a CCArGG box are involved in the cell type-specific expression of the human muscle dystrophin gene.

The muscle-specific promoter of the dystrophin gene is active in skeletal, cardiac, and smooth muscles and is specifically stimulated during differentiation of myoblasts into multinucleated myotubes. An 850-base pair (bp) DNA fragment upstream from the cap site is able to confer a partial muscle specificity to a reporter gene. The region between -850 and -140 bp includes nonspecific negative and positive regulatory sequences. A continuous stretch of 140 bp upstream from the cap site exhibits a striking conservation between rodents and human (93% homology) and still retains muscle preference of expression. It contains two putative binding sites for factors involved in regulation of other muscle-specific genes, a CCArGG box and an E box. This latter element, however, is unable to confer the ability to be transactivated by MyoD1 to the dystrophin promoter. The -140-bp promoter fragment exhibits antagonist effects contributed by one inhibiting sequence (nucleotide -140/-96), active in all cell types, and one activating region, from nucleotide -96 to the cap site, sufficient to confer a muscle preference of expression, in which the CCArGG box seems to play a major role.