The positions of the several sea urchin histone genes on the eukaryotic fragments of the chimeric plasmids pSp2 and pSp17 have been mapped relative to the Eco RI and Hind III restriction endonuclease sites on the plasmids. Two principal mapping methods using the electron microscope have been used: (a) the R-loop procedure and a new modification thereof to map the genes on duplex DNA; (b) the gene 32-ethidium bromide technique to visualize RNA-DNA hybrids on single strands of DNA. It is known that there are two histone genes, H3 and H2A, on pSp17. There are two Eco RI sites at the two junctions of the procaryotic segment with the eucaryotic segment on the plasmid. We show, by an electron microscope method, that for H2A, with a length of 0.52 kilo-bases (kb), one end of the gene is situated 0.02 to 0.03 kb from one RI site, and that there is a Hind III site within this gene at about 0.13 kb from the end proximal to the RI site. The distance from the other RI site to the proximal end of the other gene, H3, is about 0.19 kb. There are three histone genes, H2B, H1, and H4, on pSp2. The H2B gene is situated about 0.42 kb from the RI site closest to the procaryotic Hind III site, whereas the H1 gene maps at 0.76 kb from the other RI site of this plasmid. The H4 gene lies between H2B and H1. The measured lengths of genes and spacers agree with the values previously observed in circular molecules. When double-stranded plasmid DNA is incubated with histone mRNA in a high-formamide solvent, both the rate and the extent of R-loop formation increase as the incubation temperature is raised up to a temperature just below that at which strand dissociation of the duplex DNA occurs.
|Number of pages||9|
|Publication status||Published - 1977|
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