RECENT DATA OBTAINED BY PULSED-FIELD GEL-ELECTROPHORESIS SUGGEST 2 TYPES OF DOUBLE-STRAND BREAKS

The temporal evolution of unrejoined and misrejoined DNA double-strand breaks (DSBs) produced by high doses (80-160 Gy) of X rays has been e stimated using pulsed-field gel electrophoresis (PFGE) (Lobrich et at, Proc. Natl, Acad. Sci. USA 92, 12050-12054, 1995). We attempted to fi t these data to three models. An RBM (''Revell binary misrejoining'') model, based on the usual repair-misrepair and lethal-potentially leth al models, appears to be inconsistent with the data, The main discrepa ncies are the following: (1) The RBM model predicts that 90% of the mi srejoined DSBs form by the time 75% of the DSBs have disappeared, whil e the data indicate that only 50% are formed by this time; and (2) the model predicts an increasing fraction of DSBs misrejoined at 160 Gy c ompared to 80 Gy, while the data support approximately equal fractions misrejoined. These discrepancies are alleviated in the Sax subset (SS ) and Revell subset (RS) models, In the SS and RS models, two types (o r subsets) of DSBs exist: those that are active in misrejoining and th ose that are not, In the SS model, active DSBs misrejoin by the breaka ge-and-reunion mechanism described by Sax; in the RS model, active DSB s either repair, or misrejoin according to the complete exchange misre joining mechanism described by Revell, Both models are consistent with the data set considered. (C) 1998 by Radiation Research Society.