Nuclease sensitivity of alpha-fetoprotein, metallothionein-1, and immunoglobulin gene sequences in mouse during development.

The production of alpha-fetoprotein (AFP) and metallothionein-1 (MT-1) in mouse tissues follows a well-defined developmental pattern. The genes for these proteins are highly transcribed in embryo liver but transcribed at a very low rate in adult liver and in brain at all stages of development. A dot hybridization procedure was defined for quantitative screening for AFP, MT-1, immunoglobulin, and satellite DNA sequences to determine the relative degree of micrococcal nuclease sensitivity of these DNA sequences in fetal, newborn, and adult liver and brain, and the visceral yolk sac of the embryo. It was found that, for the DNA sequences assayed, three distinct chromatin conformations exist. DNA that does not code for protein (satellite DNA) was highly resistant to nuclease cleavage. DNA that codes for protein, but is not available for transcription (unrearranged immunoglobulin (C mu) genes in brain, liver, and yolk sac) was fourfold more sensitive to cleavage than were satellite DNA sequences. A further sevenfold increase in nuclease sensitivity was detected in genes actively being transcribed (MT-1 and AFP genes in embryo liver). Quiescent MT-1 and AFP genes were intermediate in nuclease-sensitivity between active genes and unrearranged C mu genes. These data indicate that MT-1 and AFP genes are permanently established in a nuclease-sensitive chromatin conformation early in liver development, and that conformation is maintained regardless of the degree of transcription of the genes. A second, reversible change in chromatin structure occurs in step with changes in the degree of developmentally regulated expression of AFP and MT-1 genes.