Evidence for contamination of origin DNA isolated after an in vivo treatment of mammalian cells with 4,5',8-trimethylpsoralen. 
We have studied the effect of in vivo treatment with trioxsalen on DNA replication in mammalian cells.  In vitro cultured bovine liver cells were exposed to two or four cycles of treatment with 45 microM trioxsalen followed by irradiation with long-wave ultraviolet light.  Thymidine incorporation was reduced by about 95% during the first hour after a double treatment.  A large proportion of the label was released in alkaline sucrose gradients as a low molecular weight fraction (average length about 500 nucleotides) which was supposed to consist of replication origins containing DNA fragments.  From the relative quantities of this DNA obtained at different times of the S phase we concluded that it contains a considerable but not precisely determinable proportion of non-origin DNA.  We also find that the fraction is contaminated by a large excess of non-replicating bulk DNA.