| The mechanics of gene transfer in Caulobacter differs from that of the more conventional system seen in the Enterobacteriaceae. The mating process in this genus takes place exclusively on solid rather than liquid medium. We could not demonstrate a natural gradient of transmission of gene transfer and the number of recombinant colonies on the selective plates suggested that mating occurred after dilutions of the mating mixture were made. Our attempts at interrupted mating failed, even though the experimental procedures designed took into consideration the probability that mating in this genus occurs only on solid media. It is possible that there is a constant breakage and reformation of the mating bridge while on the plate. Treatment of the mating mixture with pronase did not inhibit recombinant formation on the plates suggesting that the mating bridge is not protein in nature. In the process of plasmid RP1 transfer in Caulobacter, we demonstrated that only the stalked stage of the Caulobacter cell population participated in the transfer. The mechanics of plasmid RP1 and chromosomal gene transfer in Caulobacter were similar. However, when the plasmid was transferred from other genera like Pseudomonas aeruginosa or Escherichia coli CSH29 R('+) (presence of plasmid RP1), preincubation of the mating mixture enhanced efficiency of transfer. On further examination, we found that the donor R('+) cells were producing a soluble factor which could act on the recipient cells to make them more receptive to the plasmid. This soluble factor is heat labile, sensitive to pronase while insensitive to DNAase, and can be precipitated out by 50% ammonium sulfate. Upon fractionation by column chromatography, three activity peaks were seen, all of which did not correspond to the column fractions which gave the maximum absorbance at 280nm. The probable molecular weight of this protein factor is less than 100,000. |
