Absence from the mouse genome of DNA sequences related to the globin gene family

Reduced-stringency DNA reassociation: sequence specific duplex formation

Reduced-stringency DNA reassociation conditions allow low stability duplexes to be detected in prokaryotic, plant, fish, avian, mammalian, and primate genomes. Highly diverged families of sequences can be detected in avian, mouse, and human unique sequence dNAs. Such a family has been described among twelve species of birds; based on species specific melting profiles and fractionation of sequences belonging to this family, it was concluded that permissive reassociation conditions did not artifactually produce low stability structures (1). We report S1 nuclease and optical melting experiments, and further fractionation of the diverged family to confirm sequence specific DNA reassociation at 50 degrees in 0.5 M phosphate buffer.

A direct estimate of the number of human gamma-globin genes

The number of genes specifying human gamma-globin has been determined directly by hybridization of complementary DNA to total human DNA. The complementary DNA was enriched in sequences specific for gamma-globin genes by transcribing globin mRNA isolated from fetal reticulocytes with viral reverse transcriptase, and collecting the material which does not back-hybridize to adult globin mRNA. When hybridized in cDNA excess to DNA, very similar values are found for gamma-gene number as for beta-gene number, suggesting two or at most three gamma-globin genes per haploid human genome. This indicates that the non-Mendelian ratios of gamma-chain mutants found in heterozygotes are due to transcriptional or post-transcriptional regulation rather than to gene dosage. The number of each major human globin gene has now been determined directly by molecular methods.

Mouse globin gene expression in erythroid and non-erythroid tissues

Using cDNA hybridization, mouse globin RNA sequences can be detected in the nuclear and cytoplasmic RNA not only from erythroid cells (fetal liver and reticulocytes), but also in low amounts in non-erythroid tissues (adult brain and liver, and cultured lymphoma, untransformed, and transformed fibroblast cell lines). For the nuclear RNAs, melting curve, density, and size determinations on the hybrids confirmed the presence of an RNA species indistinguishable by cDNA hybridization from reticulocyte globin mRNA. Thus the genes coding for alpha- and beta-globin, proteins thought to be found only in a restricted range of differentiated cell types, may nonetheless be transcribed at very low levels in all mouse cells. The proportion of the globin RNA sequences in the nuclear RNA that are found associated with poly(A) sequences does not vary markedly from one tissue or cell line to another. In erythroid cells, however, the proportion of the cellular globin RNA sequences that are found in the cytoplasm is very much higher than in non-erythroid cells. This suggests the existence of messenger-specific nuclear post-transcriptional control mechanisms which do not use poly(A) as a signal, and which allow the cytoplasmic accumulation of globin mRNA only in erythroid tissues.