A. Chromosomes of the indicated InvA mutants, grown in minimal medium (MM) or for 1, 2 or 3 hours in LB, were restricted with the NotI enzyme and fragments were separated by PFGE. A. Ethidium bromide stained gel with NotI restricted chromosomes from InvA recB and InvA recB ruvAB mutants. Fragment sizes are indicated on the left. The star indicates the position of migration of the 171 kb DNA fragment formed by fork breakage and DNA degradation. B. Southern blot of the gel shown in A using the rrnC promoter probe, both the intact 208 kb NotI fragment and fragments of smaller sizes hybridize with the probe (the minor hybridization with the 193 kb Not1 restriction fragment may result from co-migration of broken DNA with this fragment). C. Southern blot made with the gel shown in A, using the rrnC terminator probe, only the intact 208 kb NotI fragment hybridizes with the probe. D. Schematic representation of the different DNA fragments. The triangles represent the two rrn operons as indicated, the black circle represents oriC and the bars above the lines show the positions of the probes. E. For each mutant, Southern hybridizations of 3 to 6 gels were quantified, and the percentage of hybridized DNA that remains in wells, that migrates at the 208 kb position and that migrates at the 171 kb position were calculated. For clarity, only the percentages of migrating DNA are shown here (see for complete results); light grey, 208 kb fragment (intact), dark grey, 171 kb fragment (resulting from fork breakage and DNA degradation up to rrnC). Vertical bars indicate standard deviations.