Summary: A Transformant of carrying a recombinant plasmid, pLP11 (9·5 MDa), on which the penicillinase gene () and kanamycin resistance gene () were located was subjected to mutagenesis, and a mutant plasmid (9·5 MDa; ), designated pTRA 117, obtained. A transformant of carrying pTRA117 could grow at 63 °C in medium containing kanamycin, whereas a transformant carrying pLP11 could not. Although pTRA117 was deteeted as covalently closed circular (ccc) DNA when it was extracted from transformants cultured at 48 °C, it was integrated into the host chromosome when the culture temperature was shifted up to 63 °C. If the culture temperature was lowered to 48 °C from 63 °C, a new plasmid (10·7 MDa; ), designated pTRZ117, could be detected as ccc DNA, the size of this plasmid suggested that it was pTRA117 plus a 1·2 MDa DNA fragment of the host chromosome, and this was confirmed by Southern hybridization. pTRZ90 (7·9 (7·9 MDa; kan) was constructed from pTRZ117 by the deletion of a 2·8 MDa DNA fragment that contained . Fresh transformants of that carried either pTRZ117 or pTRZ90 could grow at 65 °C.


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