Gamma rays and bleomycin nick DNA and reverse the DNase I sensitivity of beta-globin gene chromatin in vivo.

The active beta-globin genes in chicken erythrocytes, like all active genes, reside in large chromatin domains which are preferentially sensitive to digestion by DNase I. We have recently proposed that the special structure of chromatin in active domains is maintained by torsional stress in the DNA (Villeponteau et al., Cell 39:469-478, 1984). This hypothesis predicts that nicking of the DNA within any such chromosomal domain in vivo will relax the DNA and lead to loss of the special DNase I-sensitive state. Here we have tested this prediction by using gamma irradiation and bleomycin treatment to cleave DNA within intact chicken embryo erythrocytes. Both treatments cause reversal of DNase I sensitivity. Moreover, reversal occurs at approximately one nick per 150 kilobase pairs for both agents despite their entirely unrelated modes of cell penetration and DNA attack. These results suggest that the domain of DNase I sensitivity surrounding the beta-globin genes comprises 150 kilobase pairs of chromatin under torsional stress and that a single DNA nick in this region is sufficient to reverse the DNase I sensitivity throughout the entire domain.     

Albumin and alpha-fetoprotein gene transcription in rat hepatoma cell lines is correlated with specific DNA hypomethylation and altered chromatin structure in the 5'' region.

We examined DNA methylation and DNase I hypersensitivity of the alpha-fetoprotein (AFP) and albumin gene region in hepatoma cell lines which showed drastic differences in the level of expression of these genes. We assayed for methylation of the CCGG sequences by using the restriction enzyme isoschizomers HpaII and MspI. We found two methylation sites located in the 5' region of the AFP gene and one in exon 1 of the albumin gene for which hypomethylation is correlated with gene expression. Another such site, located about 4,000 base pairs upstream from the AFP gene, seems to be correlated with the tissue specificity of the cells. DNase I-hypersensitive sites were mapped by using the indirect end-labeling technique with cloned genomic DNA probes. Three tissue-specific DNase I-hypersensitive sites were mapped in the 5' flanking region of the AFP gene when this gene was transcribed. Similarly, three tissue-specific DNase I-hypersensitive sites were detected upstream from the albumin gene in producing cell lines. In both cases, the most distal sites were maintained after cessation of gene activity and appear to be correlated with the potential expression of the gene. Interestingly, specific methylation sites are localized in the same DNA region as DNase I hypersensitive sites. This suggests that specific alterations of chromatin structure and changes in methylation pattern occur in specific critical regulatory regions upstream from the albumin and AFP genes in rat hepatoma cell lines.     

Saccharomyces cerevisiae positive regulatory gene PET111 encodes a mitochondrial protein that is translated from an mRNA with a long 5'' leader.

The yeast nuclear gene PET111 is required specifically for translation of the mitochondrion-coded mRNA for cytochrome c oxidase subunit II. We have determined the nucleotide sequence of a 3-kilobase segment of DNA that carries PET111. The sequence contains a single long open reading frame that predicts a basic protein of 718 amino acids. The PET111 gene product is a mitochondrial protein, since a hybrid protein which includes the amino-terminal 154 amino acids of PET111 fused to beta-galactosidase is specifically associated with mitochondria. PET111 is translated from a 2.9-kilobase mRNA which, interestingly, has an extended 5'-leader sequence containing four short open reading frames upstream of the long open reading frame. These open reading frames exhibit an interesting pattern of overlap with each other and with the PET111 reading frame.     

Expression of rat transforming growth factor alpha mRNA during development occurs predominantly in the maternal decidua.

Previous studies have shown that transforming growth factor alpha is expressed during rodent development. To establish the site(s) of transforming growth factor alpha mRNA expression during rat embryogensis, we performed in situ hybridization and Northern blot analyses on samples of embryonic and maternal tissues at various gestational ages. Our results indicate that the high levels of transforming growth factor alpha mRNA that are observed during early development are the result of expression in the maternal decidua and not in the embryo. Decidual expression appears to be induced after implantation, peaks at day 8, and then slowly declines through day 15 at which time the decidua is being resorbed. Expression of transforming growth factor alpha mRNA is highest in that region of the decidua adjacent to the embryo and is low or nondetectable in the uterus, placenta, and other maternal tissues. The developmentally regulated expression of transforming growth factor alpha mRNA in the decidua, together with the presence of epidermal growth factor receptors in this tissue, suggests that transforming growth factor alpha stimulates proliferation locally through an autocrine mechanism. Since epidermal growth factor receptors are present in the embryo and placenta, transforming growth factor alpha produced in the decidua may also act on these tissues through paracrine or endocrine mechanisms.