The composition of coding joints formed in V(D)J recombination is strongly affected by the nucleotide sequence of the coding ends and their relationship to the recombination signal sequences.

V(D)J recombination proceeds in two stages. Precise cleavage at the border of the conserved recombination signal sequences (RSSs) and the coding ends results in flush double-stranded signal ends and coding ends terminating in hairpins. In the second stage, the signal and coding ends are processed into signal and coding joints. Coding ends containing certain nucleotide homopolymers affect the efficiency of V(D)J recombination. In this study, we have tested the effect of small changes in coding-end nucleotide composition on the frequency of coding- and signal joint formation. Furthermore, we have determined the sequences of coding joints resulting from recombination of coding ends with different compositions. We found that the presence of two T nucleotides 5' of both RSSs, but not a single T, reduces the frequency of signal joint formation, i.e., interferes with the cleavage stage of V(D)J recombination. However, coding-joint processing is sensitive even to a single T. Both the sequence of the coding ends and the particular RSS (12-mer or 23-mer) with which the coding end is associated affect the final composition of the coding joints. Thus, the presence of P nucleotides, the conservation of one undeleted coding end, the formation of joints without any deletions, and the template-dependent insertion of nucleotides are strongly influenced by the coding-end nucleotide composition and/or RSS association. The implications of these results with respect to the processing of coding ends are discussed.