Length and sequence heterogeneity of the histone gene repeat unit of the sea urchin, S. purpuratus.

Histone gene repeats in S. purpuratus are shown to be of variable length and sequence. Two recombinant plasmids containing the full-length 6.3 kb histone repeat unit are found to differer in length at two sites in the repeating structure and in the occurrence of two restriction enzyme recognition sites. Variation in repeat length is also demonstrated in the unfractionated DNA of five sea urchins and in a sample of DNA enriched for histone gene sequences by density gradient methods. The repeats in each individual are of a very limited number of major classes, which may differ from one another in overall length or in distribution and presence of particular restriction enzyme sites. Variations are found to occur at many regions of the repeat; some have been mapped specifically to spacer regions. Repeats may differ dramatically from individual to individual since there is no one type of repeat class common to all, although the absolute length differences of the repeats that are found are small.