Final answer:
Surface exclusion proteins encoded by the F plasmid block contact between the Tip protein of the pilus in F+ E. coli cells to prevent unnecessary transfer of the already possessed F plasmid during bacterial conjugation.
Step-by-step explanation:
The protein that blocks contact between the Tip protein of the pilus between two F positive (F+) cells is known as surface exclusion protein. This protein is encoded by the F plasmid, or fertility factor, in E. coli. The surface exclusion protein ensures that conjugation, the process where one bacterium transfers genetic material to another through direct contact, typically does not occur between two F+ cells, preventing the transfer of an F plasmid between cells that already possess it.
During conjugation, an F+ cell, which contains the F plasmid, extends its F pilus to make contact with an F- (recipient) cell. This establishes a bridge through which the F plasmid can be transferred. Once the F plasmid enters the F- cell, the plasmid replicates, converting the recipient into another F+ cell capable of transferring the plasmid further. The surface exclusion proteins prevent this process from happening between two cells that are both F+, thus conserving energy and resources by avoiding unnecessary plasmid transfer.