Novel system for recognizing and eliminating foreign DNA in Pseudomonas putida

Derivatives of pQSR49 (R1162::Tn1) containing cloned fragments of Escherichia coli chromosomal DNA are stable in E. coli but unstable in Pseudomonas putida. Similar derivatives containing P. putida chromosomal DNA are stable in both species. Instability is a consequence of plasmid loss during growth and is not due to death or inhibition of growth of plasmid-containing cells. Average copy numbers per cell of the unstable hybrid plasmids are similar to that of pQSR49, indicating that instability is not the result of reduced replication of plasmid DNA. A plasmid unrelated to pQSR49, RK2, becomes unstable in the presence of the unstable hybrids but only when it also contains foreign DNA. The data suggest an inducible mechanism in P. putida active in the elimination of plasmid-borne foreign DNA from the cell.