Insertion into coding sequences of nbs. Transcription in the mutant allele initiates in the region downstream of the w gene carried in the inserted P{5'EPgy2.SM9} element. This downstream region is derived from sequence that is located 3' to the endogenous w locus. In addition, three introns, with canonical splice donor and acceptor sequences, are spliced out of this region in the mutant transcript. Two of these introns are located in the region downstream of w, but the third is within the P-element 5' end, which in the mutant is transcribed in the direction opposite to that of P\T transcription. The terminal 3bp of the P-element end form a start codon that is in-frame with the nbs coding sequence.
Imprecise excision of the progenitor insertion. The 3' P-element end, all of the y[+] marker and much of the w[+] marker has been removed, with nucleotides 1-3019 of the original insertion remaining. Transcription in this mutant initiates within the P-element, just downstream of the 3' end of the w sequences. Translation is predicted to begin at the last three nucleotides of P-element sequence and continue in-frame into nbs sequences. In the predicted translation product, the first residue of the first BRCT domain is changed from Leu to Met, but the domain is otherwise intact. The nbs N-terminal FHA domain is missing in this product as the insertion occurs downstream of the sequence coding this domain.
nbs1/nbsSM9 larvae are hypersensitive to gamma irradiation compared to controls.
The total percentage of progeny representing repair in studies of excision and repair events of P{hswa} is similar in nbs1/nbsSM9 flies and wild-type controls. However, the fraction of repair events that are attributed to completed synthesis-dependent strand annealing (SDSA) is very low in nbs1/nbsSM9 flies compared to wild type. In the repair events that are not due to completed SDSA, the number of repair events that do not have evidence for synthesis is increased in the mutant flies compared to wild type, although synthesis is not entirely abolished. Synthesis tracts are significantly shorter in the mutant flies than in wild type.
Studies of DNA double-stranded break repair indicate that frequencies of single-strand annealing (SSA), deletions and imprecise non-homologous end joining (NHEJ) are not significantly different between nbs1/nbsSM9 and wild-type larvae.
