Evolution of TRG clusters in cattle and sheep genomes as drawn from the structural analysis of the ovine TRG2@ locus.

The availability of genomic clones representative of the T-cell receptor constant gamma (TRGC) ovine genes enabled us to demonstrate, by fluorescent in situ hybridization (FISH) on cattle and sheep metaphases, the presence of two T-cell receptor gamma (TRG1@ and TRG2@) paralogous loci separated by at least five chromosomal bands on chromosome 4. Only TRG1@ is included within a region of homology with human TRG locus on chromosome 7, thus TRG2@ locus appears to be peculiar to ruminants. The structure of the entire TRG2@ locus, the first complete physical map of any ruminant animal TCR gamma locus, is reported here. The TRG2@ spans about 90 kb and consists of three clusters that we named TRG6, TRG2, and TRG4, according to the constant genes name. Phylogenetic analysis has highlighted the correlation between the grouping pattern of cattle and sheep variable gamma (TRGV) genes and the relevant TRGC; variable (V), joining (J), and constant (C) rearrange to be found together in mature transcripts. The simultaneous results on the TRG2@ locus molecular organization in sheep and on the phylogenetic analysis of cattle and sheep V expressed sequences indicate that at least six TRG clusters distributed in the two loci are present in these ruminant animals. The inferred evolution of TRG clusters in cattle and sheep genomes is consistent with a scenario where a minimal ancient cluster, containing the basic structural scheme of one V, one J, and one C gene, has undergone a process of duplication and intrachromosomal transposition.