Final answer:
Surface exclusion proteins are encoded by the F plasmid and prevent DNA transfer between F+ cells by inhibiting the formation of a stable mating pair during the process of bacterial conjugation.
Step-by-step explanation:
The protein that blocks the transfer of DNA between F positive (F+) cells in bacterial conjugation is called a surface exclusion protein, which is encoded by the F plasmid itself. When an F+ cell contains the F plasmid, it can form a conjugation pilus to transfer a copy of the plasmid to an F- (recipient) cell, converting it into an F+ donor cell. However, to prevent the redundant transfer of the F plasmid between F+ cells, these cells express surface exclusion proteins within the inner membrane that prevent the formation of a stable mating pair, thus inhibiting the conjugation process between two F+ cells. This ensures that the energy and resources are conserved for transfers to F- cells, which do not yet possess the F plasmid.