Histone gene clusters of the newt notophthalmus are separated by long tracts of satellite DNA

Histone gene number in relation to C-value in amphibians

We have compared the number of copies of sequences complementary to a cloned Xenopus histone H4 coding sequence in the genomes of Xenopus, Triturus and Ambystoma, amphibian species with widely different C-values (3, 23 and 38pg DNA/haploid genome respectively). Quantitative autoradiography indicates that H4 sequence constitute a greater proportion of the genome the larger that genome is. Measurement of the absolute copy-number by reassociation kinetic analysis indicated 47 +/- 10, 636 +/- 21 2685 +/- 349 copies per haploid genome each in Xenopus, Triturus and Ambystoma respectively. Whilst this confirms a trend of increasing copy-number with increasing C-value, the two are not directly proportional and some other factors must contribute to determining the number of copies of these genes.

Histone genes are located at the sphere loci of newt lampbrush chromosomes

In situ hybridization experiments on mitotic and lampbrush chromosomes show that histone genes and an associated 222 bp repeated sequence, satellite 1, are located at or near the sphere loci of chromosomes 2 and 6 of the newt Notophthalmus viridescens. During the lampbrush chromosome stage, transcripts of the histone genes and of satellite 1 occur on loops associated with the spheres. Histones genes are located at sites on the mitotic chromosomes of Triturus cristatus and T. alpestris which are consistent with the known positions of spheres on their lampbrush chromosomes.

Transcripts from both strands of a satellite DNA occur on lampbrush chromosome loops of the newt Notophthalmus

We have isolated genomic DNA clones from the newt Notophthalmus which contain one or more copies of a 222 bp tandemly repeated sequence (satellite 1). Satellite 1 occurs in the pericentromeric heterochromatin of all chromosomes and at the sphere loci of chromosomes 2 and 6. Lampbrush chromosome loops associated with the spheres contain transcripts of satellite 1, as shown by in situ hybridization. The matrix on these loops consists of multiple thin--thick transcription units, most but not all of which have the same polarity within a given loop. By in situ hybridization with single-stranded probes we show that transcription units of opposite morphological polarity contain transcripts of opposite strands of satellite 1. Satellite 1 occurs as the spacer between clusters of histone genes. We postulate that transcription initiates at a histone gene promoter, fails to terminate at the end of the gene and continues without interruption into the adjacent satellite DNA. In this way, long transcripts are produced which contain both histone and satellite 1 sequences. A general failure of termination signals might explain the unusually long transcripts of lampbrush chromosomes.