Summary: We have constructed four deletion derivatives of the cloned gene. Plasmid pDD1, in which the last 10 amino acids of the DnaK protein have been replaced by three different amino acids derived from the pBR322 vector, was as effective as plasmid pKP31, from which it was derived, in restoring the ability of a null mutant, BB1553, to plate λ phage and to grow at high temperatures. The other three mutations, involving much larger deletions of the gene, did not restore the ability to plate λ phage or the ability to grow at high temperatures. Plasmid pKUC2, which contains the whole gene and its promoters, was capable of restoring the ability of BB1553 to plate λ phage but, surprisingly, it did not restore the ability to grow at high temperatures, even though it was shown that the DnaK protein was efficiently expressed in these cultures. By transposon mutagenesis and sub-cloning, we have shown the presence of a second gene in plasmid pKP31 which is required for high-temperature growth of BB1553. This gene, which we call , is presumably also defective in the null mutant BB1553. We have also demonstrated that the inability of K756 to grow above 43.5 °C is complemented by sub-clones which contain the gene, but not by plasmid pKUC2.


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