We have investigated transcription of the histone gene cluster on lampbrush chromosomes of the newt Notophthalmus viridescens. Clusters of the five histone genes are separated by long tracts of a DNA repeat called satellite 1. Previous in situ hybridization results demonstrated the presence of histone gene coding regions, intergene spacers, and flanking satellite repeats in giant transcription units on lampbrush chromosomes. These results suggested a model in which transcription initiates at histone gene promoters and continues through the rest of the gene cluster into satellite repeats. The readthrough transcription model predicts that spacer regions upstream of the first promoter in the gene cluster should be absent from nascent transcripts on chromosome loops. We have used such upstream spacer probes for in situ hybridization to lampbrush chromosomes. Contrary to our expectation, the results show that upstream spacer regions are transcribed, and indicate that transcription often initiates upstream of the first histone gene promoter. The relationship of giant transcription units in oocyte nuclei to cytoplasmic histone mRNA is discussed.
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