Discovery of the fertility factor F

In 1953, William Hayes discovered that in the above types of "crosses" the conjugating parents acted unequally (later we shall see ways to demonstrate this). It seemed that one parent (and only that parent) transferred some of or all its genome into another cell. Hence one cell acts as donor, and the other cell as a recipient. This is quite different from eukaryotic crosses in which parents contribute nuclear genomes equally.

MESSAGE The transfer of genetic material in E. coli conjugation is not reciprocal. One cell, the donor, transfers part of its genome to the other cell, which acts as the recipient.

By accident, Hayes discovered a variant of his original donor strain that would not produce recombinants on crossing with the recipient strain. Apparently, the donor-type strain had lost the ability to transfer genetic material and had changed into a recipient-type strain. In working with this "sterile" donor variant, Hayes found that it could regain the ability to act as a donor by association with other donor strains. Indeed the donor ability was transmitted rapidly and effectively between strains during conjugation. A kind of "infectious transfer" of some factor seemed to be taking place. He suggested that donor ability is itself a hereditary state, imposed by a fertility factor (F). Strains that carry F can donate, and are designated F+. Strains that lack F cannot donate and are recipients, designated F".

We now know much more about F. It is an example of a small, nonessential circular DNA molecule called a plasmid that can replicate in the cytoplasm independent of the host chromosome. Figures 5-6 and 5-7 show how bacteria can transfer plasmids such as F.

Bacterial Conjugation
Figure 5-6 Bacteria can transfer plasmids (circles of DNA) through conjugation. A donor cell extends one or more projections—pili—that attach to a recipient cell and pull the two bacteria together. [Oliver Meckes/MPI-Tubingen, Photo Researchers.]

5.2 Bacterial conjugation

Figure 5-7 Conjugation.

(a) During conjugation, the pilus pulls two bacteria together.

(b) Next, a bridge (essentially a pore) forms between the two cells. A single-stranded copy of plasmid DNA is produced in the donor cell and then passes into the recipient bacterium, where the single strand, serving as a template, is converted to the double-stranded helix.

Bacterial chromosome

Bacterial chromosome

Figure 5-7 Conjugation.

(a) During conjugation, the pilus pulls two bacteria together.

(b) Next, a bridge (essentially a pore) forms between the two cells. A single-stranded copy of plasmid DNA is produced in the donor cell and then passes into the recipient bacterium, where the single strand, serving as a template, is converted to the double-stranded helix.

Bill Hayes Discovery Fertility

Recipient F -

Bacteria Fertility Factor

Recipient F -

The F plasmid directs the synthesis of pili, projections that initiate contact with a recipient (Figure 5-6) and draw it closer. The F DNA in the donor cell makes a single-stranded copy of itself in a peculiar mechanism called rolling circle replication. The circular plasmid "rolls," and as it turns, it reels out the single-stranded copy like fishing line. This copy passes through a pore into the recipient cell, where the other strand is synthesized, forming a double helix. Hence a copy of F remains in the donor and another appears in the recipient, as shown in Figure 5-7. Note in the figure that the E. coli genome is depicted as a single circular chromosome. (We will examine the evidence for this later.) Most bacterial genomes are circular, a feature quite different from eukaryotic nuclear chromosomes. We shall see that this feature leads to many idiosyncrasies of bacterial genetics.

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