FIG. 4.

Chromosome cleavage following loss of the EcoRI RM gene complex. Cultures of three E. coli strains, AB1157 (rec⁺), JC5519 (recB21 recC22), and BIK3686 (recC1002), carrying pIK172 (pSC101Ts, EcoRI r⁺ m⁺, Ap^r) or pIK173 (its r⁻ version) were transferred from 30 to 42°C as described in the legend to Fig. 1. The cells were mixed with 2,4-dinitrophenol to block energy metabolism at the indicated time intervals (in hours) after the temperature shift and were treated as described previously (33). DNA was electrophoresed through 1.0% agarose gel at 15°C in 45 mM Tris-borate–1.25 mM EDTA at 165 V with a pulse time of 50 s for 24 h using hexagonal electrodes in a Pharmacia LKB apparatus. Lane M contains Saccharomyces cerevisiae chromosomes.