Hfr-Mediated Genetic Transfer in E. coli

In the mating of Hfr x F- the LAST thing to get across into the F- is the F-factor, which happens only infrequently because thermal aggitation or an inappropriate stirring causes the pilus to break and only some of the "early" Hfr genes get transferred. I suspect you can understand why this is called an interrupted mating.

See if you can follow this series of such an interrupted mating:

Here are a couple of E.coli at play. I have put a few notes on the Hfr's chromosome: there is an arrowhead denoting the origin "o", and after it are the early genes (a, b, c..); and going all the way around the circle you see the "late" genes (y, z) and finally the F. Notice that the "early" genes transfer - well, early! (Another thing to remember is that F (and "o") can integrate into the chromosome at seemingly any point on the chromosome, and in either direction. Thus after Bill Hayes discovered the first Hfr (cleverly called HfrH), other people got on the bandwagon and isolated many others of varied origins and directions. You know of course, that Hfr stands for a cell that gives "high frequency recombination" when used in a mating experiment.) (Also remember that F is a plasmid, like many others, and each type has its own pili. However, F can integrate AND can "mobilize" the entire chromosome.)

Now for a mental mnemonic: imagine that the Hfr's chromosome is an arrow. You see a molecule of DNA polymerase (the black dot) sitting just of the arrowhead. This particular type of DNA polymerase is not the usual kind used when cells divide. This type is only for mating, and its genes are in the F-factor: its "start" site is always "o", the tip of the arrow in Hfr's. (That fact was part of my PhD thesis.) What is more is that it makes only a single-stranded copy of the Hfr DNA. It is that ssDNA to works its way through the pilus. At 37C, it takes about 90 minutes to transfer (or at 35C about 100 minutes) to get all of the genes through - all the way to and including the F. (See the polymerase move down the "arrow"?) Usually, however, the process is interrupted. Adelberg finally figured that if the mating were to happen on the surface of agar, few interruptions ought to occur and he got his F- recipients to get as much as three rounds of the Hfr's DNA. Of course, the F- became at least F+.

Here you see the F- after receiving only a portion of the Hfr's chromosome, and then interruption - probably caused intentionally by the researchers, who sadistically turned on the blender in which the cells were playing.

Another mnemonic: we can diagrammatically align the Hfr's fragment of DNA with the F-'s chromosome.

At this point, the F- cell's recombinases can come into use:

In this instance, the Hfr's a and b genes were crossed into the F-'s chromosome. Those that were crossed out plus those Hfr genes not crossed in rather soon are destroyed or diluted out as the cell goes on through several divisions. (Upper and lower case are only used to denote whose genes are where, and do not denote dominance.)

Now are you totally confused?