STRUCTURE OF EXTRACHROMOSOMAL CIRCULAR DNAS EXCISED FROM T-CELL ANTIGEN RECEPTOR ALPHA-CHAIN AND DELTA-CHAIN LOCI

Small polydisperse circular (spc) DNA was isolated from mouse thymocytes, fragmented by HindIII digestion and cloned into the vector. Sixty DNA clones were randomly selected from the 10,400 phage library. The average size of insert was one-fifth of the original circular molecule. Twenty spc-DNA clones were homologous to DNA probes derived from T-cell antigen receptor (TCR) .alpha.-chain loci. We have characterized nine clones by DNA sequencing; they contain new germline sequences of the TCR .alpha.-chain variable (V.alpha.) and joining (J.alpha.) gene segments and the products out of the recombinatin of a V.alpha. with a J.alpha. gene segment. An additional four spc-DNA clones carried a new rearranging gene of the TCR .delta.-chain that is located between V.alpha. and J.alpha. genes. at least nine of 60 DNA clones carried the recombination junction of a heptamer-heptamer head-to-head structure expected from an excised product of V-J joining. This shows that most extrachromosomal circular DNAs in the thymus are formed by a sequence-dependent recombination mechanism. We suggest that a functional T-cell receptor V.alpha. gene can be constructed by somatic random rearrangements through successive looping-out, excision and deletion.