Lymphoid V(D)J recombination. Functional analysis of the spacer sequence within the recombination signal.

The V(D)J recombination reaction is directed by a pair of signal sequences, each consisting of a palindromic heptamer, an A/T-rich nonamer, and an intervening spacer sequence of 12 or 23 base pairs. The spacer sequence previously has not been analyzed for a functional role. In this study, numerous simultaneous sequence changes have been made in the spacer of each signal to test their functional importance. All of the AT base pairs in each signal were changed to GC base pairs. This particular change is of interest because it markedly increases the energy that would be required to melt out the two strands of each signal to permit the intersignal base pairing proposed in a commonly invoked model for signal-signal interaction in V(D)J recombination. We find that changing 6 to 12 AT base pairs in the 12-signal to GC does not affect V(D)J recombination, nor does changing 11 of 23 AT base pairs in the 23-signal. Substrates with all-GC spacer sequences in both the 12- and the 23-signal also recombine at efficiencies that are not significantly reduced. These studies demonstrate that the sequences at these particular positions are not recognized by the recombinase. In addition, the data do not support models invoking signal-signal base pairing.