Specific chromosomal sites enhancing homologous recombination in Escherichia coli mutants defective in RNase H
1993
Nishitani, H. (Kyushu Univ., Fukuoka (Japan). Graduate School of Medical Science. Dept. of Molecular Biology) | Hidaka, M. | Takashi Horiuchi, T.
To clone new replication origin(s) activated under RNase H-defective (rnh(-)) conditions in Escherichia coli cells, whole chromosomal DNA digested with EcoR1 was to with a Km(r) DNA fragment and transformed into an rnh(-) derivative host. From the Km(r) transformants, eight kinds of plasmid-like DNA were obtained, each of which contained a specific DNA fragment, termed "Hot", derived from the E. coli genome. Seven of the Hot DNAs (HotA-G) mapped to various sites within a narrow DNA replication termination region (about 280 kb), without any particular selection. Because Hot DNA could not be transformed into a mutant strain in which the corresponding Hot region had been deleted from the chromosome, the Hot DNA, though obtained as covalently closed circular (ccc) DNA, must have arisen by excision from the host chromosome into which it had initially integrated, rather than by autonomous replication of the transformed species. While Hot DNA does not have a weak replication origin it does have a strong recombinational hotspot active in the absence of RNase H. This notion is supported by the finding that Chi activity was present on all Hot DNAs tested and no Hot-positive clone without Chi activity was obtained, with the exception of a DNA clone carrying the dif site.
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