SummaryIn bacteria, DNA supercoil movement is restricted to subchromosomal regions or 'domains.' To elucidate the nature of domain boundaries, we analysed reaction kinetics for ␥␦ site-specific resolution in six chromosomal intervals ranging in size from 14 to 90 kb. In stationary cultures of Salmonella typhimurium, resolution kinetics were rapid for both short and long intervals, suggesting that random stationary barriers occur with a 30% probability at approximately 80 kb intervals along DNA. To test the biochemical nature of domain barriers, a genetic screen was used to look for mutants with small domains. Rare temperaturesensitive alleles of DNA gyrase and Topo IV (the two essential type II topoisomerases) had more supercoil barriers than wild-type strains in all growth states. The most severe gyrase mutants were found to have twice as many barriers in growing cells as wild type throughout a 90 kb interval of the chromosome. We propose that knots and tangles in duplex DNA restrain supercoil diffusion in living bacteria.

Current guidelines on thrombolysis post stroke with recombinant tissue plasminogen activator (rt-PA) exclude its use where time of onset is unknown, thus denying some patients potentially beneficial treatment. Contrast enhanced perfusion computed tomography (pCT) imaging can be used together with plain CT and information on clinical deficits to decide whether or not thrombolysis should be initiated even though the exact time of stroke onset is unknown. Based on the results of pCT and CT, rt-PA was administered to two patients with unknown time of stroke onset; one of the patients also underwent suction thrombectomy. Results in both cases were excellent.

SummaryTransposition immunity is the negative influence that the presence of one transposon sequence has on the probability of a second identical element inserting in the same site or in sites nearby. A transposition-defective Mu derivative (Mud Jr1) produced transposition immunity in both directions from one insertion point in the Salmonella typhimurium chromosome. To control for the sequence preference of Mu transposition proteins, Tn10 elements were introduced as targets at various distances from an immunity-conferring Mud Jr1 element. Mu transposition into a Tn10 target was not detectable when the distance of separation from Mud Jr1 was 5 kb, and transposition was unencumbered when the separation was 25 kb. Between 5 kb and 25 kb, immunity decayed gradually with distance. Immunity decayed more sharply in a gyrase mutant than in a wild-type strain. We propose that Mu transposition immunity senses the domain structure of bacterial chromosomes.

Excision repair can be measured by using benzoylated naphthoylated DEAE cellulose (BND cellulose) to filter out DNA formed by replicative synthesis. A rapid batch method that permits analysis of many samples has been developed. Repair can be measured at growing points and in the bulk of the DNA. Using this technology, we have shown that the mutagen methyl nitronitrosoguanidine induces increased repair rates in DNA growing point regions. We have also observed that excision repair capability decreases as chick retinal tissues develop. "Glial-like" cells growing out after trypsinization of the retinas are devoid of measurable repair activity. BND cellulose chromatography can also be used to detect interruptions in DNA. Treatment of cells with methyl methanesulfonate (MMS) leads to interruptions in cellular DNA, in contrast to treatment with acetoxy acetylaminofluorene (AAAF), which is an active repair-inducing agent but which does not induce detectable numbers of breaks in cellular DNA.We conclude that there are at least two mechanisms for excision repair. The first, the MMS type, produces DNA with interruptions and inserts relatively few nucleotides. The second, the AAAF type, does not result in detectable interruptions in DNA and produces relatively long repair patches.

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