Modulation of terminal deoxynucleotidyltransferase activity by the DNA-dependent protein kinase

Rare Ig and TCR coding joints can be isolated from mice that have a targete d deletion in the gene encoding the 86-kDa subunit of the Ku heterodimer, t he regulatory subunit of the DSA-dependent protein kinase (DNA-PK). However in the coding joints isolated from Ku86(-/-) animals, there is an extreme paucity of N regions (the random nucleotides added during V(D)J recombinati on by the enzyme TdT), This finding is consistent with a decreased frequenc y of coding joints containing N regions isolated from C.B-17 SCID mice that express a truncated form of the catalytic subunit of the DNA-PK (DNA-PKCS) . This finding suggests an unexpected role for DNA-PK in addition of N nucl eotides to coding ends during V(D)J recombination, In this report, we estab lish that TdT forms a stable complex with DNA-PK Furthermore, we show that DNA-PK modulates TdT activity in vitro by limiting both the length and comp osition of nucleotide additions.