Molecular on situ hybridization analysis of sheep and goat Bac clones identifies the transcriptional orientation pf T cell receptor gamma genes on chromosome 4 in bovids

The T-cell receptor gamma chain combines with the T-cell receptor delta chain to form the heterodimer gamma/delta complex. In human and mouse, gamma/delta T cells amount to less than 5% of the peripheral blood T lymphocytes (‘gamma/delta low species’), whereas in chicken and artiodactyls, gamma/delta T cells amount to about half of the peripheral blood T cells (‘gamma/delta high species’). Recent studies in bovids have revealed the existence of two paralogous loci TRG1 and TRG2. TRG1 is located on 4q31, within a region of homology with human TRG locus on chromosome 7; while TRG2, localized on 4q15–22, appears to be unique to these ruminant animals. The entire TRG2 locus is composed of three recombinational units (cassette) named TRG6, TRG2 and TRG4 according to TRGC genes. All TRG cassettes lie in the same transcriptional orientation and are closely spaced. In each ‘V, J, C’ unit, Variable and Joining genes rearrange to be found together spliced to the relevant Constant in mature transcripts. Here, we report the cytological mapping of TRG2 locus on sheep 4q22 unique chromosome band and on homologous chromosome bands of cattle, goat and river buffalo. Moreover molecular characterization of 5' boundaries of sheep TRG2 locus and dual-colour FISH results allowed us to identify the physical order of the tightly linked TRG2 and HGF loci markers along the chromosome and establish the transcriptional orientation towards the centromere of Variable (V), Joining (J) and Constant (C) TRG2 gene segments on 4q22 band.