Directional transport and integration of donor DNA in Haemophilus influenzae transformation.

DNA transport and integration in Haemophilus influenzae transformation was studied with a plasmid clone of homologous DNA (pCML6). Our results indicate that: (i) donor DNA enters specialized membranous extensions on the cell surface, which we have termed "transformasomes"; (ii) linear DNA undergoes degradation upon exiting transformasomes; and (iii) DNA without a free end remains within transformasomes and is not degraded. By comparing the fate of label from uniformly labeled versus middle-labeled DNA, it appears that donor DNA undergoes degradation from an end prior to recombining with the chromosome. Using donor DNA with covalently closed termini (hairpin ends) prevents efficient exit from transformasomes. When one hairpin is removed, exit of donor DNA is shown to be directional from the free end, with preferential homologous integration of the 3' strand from that end.