A novel nuclease activity from Xenopus laevis releases short oligomers from 5'-ends of double- and single-stranded DNA.
BACKGROUND
Double-strand breaks in chromosomal DNA of eucaryotic cells are assumed to be repaired by mechanisms of illegitimate recombination capable of direct rejoining of the broken ends. Cell-free extracts of Xenopus laevis eggs efficiently perform these end joining reactions with any pair of noncomplementary DNA termini whose single-stranded 5'- or 3'-overhangs do not exceed a length of approximately 10 nt.


RESULTS
Using hairpin-shaped oligonucleotides that allow the construction of double-strand break termini with 5'- or 3'-overhangs of defined length and sequence we show that 5'-overhangs of more than 9-10 nt are exonucleolytically resected in the extract to produce shorter 5'-overhangs that can be metabolized in the end joining reaction. 5'-recessed ends in double-stranded DNA with 3'-overhangs of more than 2nt as well as the 5'-ends of single-stranded DNA also serve as substrates for the exonuclease activity. In all cases, oligomers of about 10 nt are released from the 5'-ends.


CONCLUSIONS
We describe here a novel 5'-exonuclease activity present in eggs from Xenopus laevis that reproducibly removes decameric oligonucleotides from 5'-ends of double- and single-stranded DNA. A possible function of this unusual activity is discussed in the context of homologous and illegitimate genetic recombination processes.
Reichenberger
S
S
Institut für Genetik der Universität zu Köln, Germany.
Brüll
N
N
Feldmann
E
E
Göttlich
B
B
Vielmetter
W
W
Pfeiffer
P
P
D000818:Animals; D002474:Cell-Free System; D004247:DNA / Q000378:metabolism; D004260:DNA Repair / Q000235:genetics; D004277:DNA, Single-Stranded / Q000378:metabolism; D005092:Exonucleases / Q000378:metabolism*; D009865:Oocytes; D013379:Substrate Specificity / Q000235:genetics; D014982:Xenopus laevis / Q000378:metabolism
D004277:DNA, Single-Stranded; D004247:DNA; D005092:Exonucleases