Molecular genetics of the S region of the murine H-2 major histocompatibility complex

The androgen-dependent C4-Slp gene is driven by a constitutively competent promoter

The androgen-dependent liver protein Slp, together with its constitutively expressed closely related isoform C4, provides a model to address the question of which minimal alteration in DNA can shut off the expression of a gene in a manner reversible by testosterone or by trans-acting mutations. Previous work indicated that sequences located at -1.9, -0.45, and -0.25 kb from the transcription start site of the C4-Slp gene played a critical role in determining its unusual functional divergence from C4. Now, using quantitatively and qualitatively controlled transfection assays in HepG2 human hepatoma cells and mouse L fibroblasts, we have observed that the C4-Slp promoter is fully effective and unhindered by upstream sequences and that the C4 promoter has a consistent albeit modest superiority. The determinant of this nearly 2-fold difference does not coincide with the sites highlighted in previous studies but lies within the most cap-site-proximal nucleotides, at positions -189 to +48. We have also established conditions for cell-free transcription of C4 and C4-Slp from plasmid and cosmid templates by using nuclear extracts from rat and mouse liver of both sexes as well as from L cells. At variance with the rat alpha 2u-globulin gene, C4-Slp transcription in vitro does not require male factors, for it is expressed as efficiently as C4 by all nuclear extracts. Further, the minimal promoter sequences required to direct accurate initiation extend not farther than the most proximal 19 nucleotides. Because L cells efficiently express transfected cosmids covering the whole C4 gene or C4/C4-Slp recombinants, as well as plasmids carrying the C4-Slp promoter, but fail to express the full C4-Slp gene, we favor a model in which the expression of the gene is modulated intragenically.

Organization of C4 and CYP21 loci in gorilla and orangutan

The standard human haplotype contains two C4 and two CYP21 loci arranged in the order C4A ... CYP21P ... C4B ... CYP21 and intercalated between the class I and class II loci of the HLA complex. The C4A gene is 22 kilobases (kb) long; the C4B gene is either 22 kb or 16 kb long. The CYP21P is a pseudogene characterized by an eight base pair (bp) deletion in exon 3 and other defects; the CYP21 is a functional gene. The standard chimpanzee haplotype is arranged in the same way as the standard human haplotype, except that both C4 genes are of the short variety; like the human gene, the chimpanzee CYP21P gene contains the 8 bp deletion. In the present study we demonstrate that a representative gorilla haplotype also consists of two short C4 genes and two CYP21 genes, neither of which, however, has the characteristic 8 bp deletion. On the other hand, the single characterized orangutan haplotype is organized in the following way: C4A ... CYP21 ... C4A ... CYP21 ... C4B ... CYP21. The first two C4 genes are of the long variety, the third gene is short. None of the defects characterizing the human CYP21P gene is present in any of the three orangutan genes. These conclusions are based on the analysis of overlapping clones isolated from cosmid libraries of the indicated species. The observed haplotype organization of the four primate species can be explained by expansion and contraction of the C4-CYP21 region through unequal homologous crossing-over, which preserves the differentiation of the C4 genes into the A and B categories but otherwise homogenizes these genes, as well as the CYP21 genes, within a given species. The 8 bp deletion in the CYP21P gene is postulated to have occurred before the separation of the lineages that led to modern humans and chimpanzees, but after the separation of these two lineages from the lineage that led to modern gorillas. The 6 kb insertion generating the long C4 gene is postulated to have occurred before the separation of the orangutan, gorilla, chimpanzee, and human lineages.