Homology and repair of UV-irradiated plasmid DNA in haemophilus influenzae

Effect of glycerol on plasmid transfer in genetically competent Haemophilus influenzae

The small plasmid pAT4 transformed at characteristically low frequencies those competent Haemophilus influenzae Rd strains that had no DNA homology with this plasmid. Transformation was increased up to 100 times, however, when the recipient cells were exposed to 30% glycerol before plating for transformants. Expression of plasmid resistance markers was then immediate. Ultraviolet irradiation experiments indicated that this large increase was due to release by the glycerol of double-stranded plasmid molecules, presumably from transformasomes. Several other plasmids exhibited the same phenomenon. Dimethylsulfoxide also stimulated plasmid transformation but lysolecithin and high concentrations of NaCl or glucose were ineffective. Glycerol did not increase the efficiency of transformation by either chromosomal DNA or linearized plasmid DNA.

Homology-facilitated plasmid transfer in Haemophilus influenzae

The 8 kbp plasmid pAT4 transformed Haemophilus influenzae Rd cells at low frequencies. Transformation was increased up to 100 times, however, when the recipient cells carried a DNA segment in either their chromosome or in a resident plasmid that was homologous to at least part of plasmid pAT4. Linearized plasmid DNA molecules did not transform cells without DNA homology; they efficiently transformed homology recipients, but only when the cuts had been made in the region of shared homology. In most cases examined the circular donor plasmid had been reconstituted from the transforming DNA; in some cases the reconstituted plasmid carried a mutation initially present in the recipient chromosome, provided the transforming plasmid had been linearized in the region of shared homology. Plasmid reconstitution was not observed in recA1 cells. We conclude that homology-facilitated plasmid transformation (transfer) is similar to that reported for Bacillus subtilis and Streptococcus pneumoniae.