The relative positions of sea urchin histone genes on the chimeric plasmids pSp2 and pSp17 as studied by electronmicroscopy.

The relative positions of the sea urchin histone genes and the spacer regions on the chimeric plasmids pS p2 and pSp17 have been mapped by hybridizing total histonemessenger RNA to single strands of the plasmid DNAs. The lengths and spacing between the several RNA:DNA duplex regions on the single strands of DNA were measured by the gene 32-ethidium bromide electron microscope mapping method. We find that the genes are interdigitated with spacer sequences of different lengths; that there are three coding sequences on pSp2, all on the same strand, with the relative order H1, H4, and B4; and that there are two coding sequences on pSp17, both on the same strand, corresponding to the messages denoted B1 and B2-B3, where B4, B1, and B2-3 are electrophoretically resolved components of histone mRNA, all of size intermediate between the larger H1 and the smaller H4 message.