Conjugation in Pseudomonas aeruginosa.

N general terms, the characteristics of the conjugation system in Pseudomonas I aeruginosa show some similarities to those of the more intensively studied Escherichia coli. Mating occurs between parents differing in the possession of a fertility factor FP. Genetic transfer is from the male FP+ or donor parent to the female F P or recipient parent. The sex factor FP is infectious in that it can be transferred from some FP+ strains to the FPstrain 1 at a frequency far in excess of that found for the transfer and recovery of chromosomal markers. Not all FP+ strains are infectious and as male forms comparable to the E. coli Hfr have not yet been found, the role of FP in transferring chromosom? is uncertain. Two main issues have conditioned our approach in this further study of pseudomonad conjugation. Firstly the nature of chromosome transfer in P. aeruginosa, and secondly the use of conjugation for the general mapping of the P. aeruginoaa chromosome. In selecting the experimental system to do this, it became obvious that there were distinct advantages in carrying out matings between F P and FP+ forms of the same strain as has been done with E. coli K12. Recombination in P. aeruginosa was first demonstrated between four unrelated strains (HOLLOWAY 1955). However, we have since shown that in the best studied of these mating combinations, 1 . P x 2.FP+, the recombination of markers is considerably influenced by DNA host-specificity differences ( HOLLOWAY and ROLFE 1964; ROLFE and HOLLOWAY 1966). As FP from strain 2 can be introduced into strain 1 by recombinants from 1 X 2 crosses which are infectious for FP (HOLLOWAY and JENNINGS 1958), such differences can be avoided by using 1 . F P x 1 .FP+ crosses. Previously, mating in P . aeruginosa was carried out by plating washed saline suspensions of the two parents onto minimal agar. The analysis of recombinants derived from such a procedure can be used to determine some aspects of linkage but such results do not enable us to draw any conclusions as to how chromosome is transferred. In this paper we will describe the application of an interrupted mating technique to P. aeruginosa to show that there is an oriented transfer of chromosome; and to discuss the use of this technique for linkage studies.