Eo: a history of a mutation.

Eighteen mouse t haplotype-carrying strains were found not to express cell-surface E molecules controlled by class II genes of the H-2 complex (= Eo strains). Northern and Southern blot analysis of these and other, non-t strains that also fail to express the E molecule, has revealed two kinds of defect. Three strains (CRO437, tw2, and presumably to) were found to transcribe the E alpha gene, but they were not able to convert the message into a functional protein. All other Eo strains fail to transcribe the E alpha gene because of a deletion encompassing the promoter region, the RNA initiation site, and the first exon. The length of the deletion is approximately 650 +/- 50 bp. These two defects closely resemble those found previously in standard inbred strains carrying the H-2f, H-2q (failure of E mRNA to be expressed functionally), H-2b, and H-2s (deletion of a part of the E alpha gene) haplotypes. In particular, the location and length of the E alpha deletion appear to be the same in the strains carrying this mutation. The E alpha deletion is in linkage disequilibrium with certain alleles at other H-2 loci in some of the strains. These observations, combined with the growing evidence that H-2 haplotypes associated with t chromosomes derive from a single ancestral haplotype, suggest that the E alpha deletion is an old mutation and that it has been disseminated in mouse populations by the t chromosomes.