Lysogenic And Lytic Cycle
6.1 Haploidy #
6.1.1 What being haploid makes easy #
Escherichia coli has one main circular chromosome and is genetically haploid, so a new loss of function is expressed at once. Populations are huge and a generation can be about twenty minutes in rich medium, so rare mutants and recombinants are recoverable by selection.
Alleles remain unpaired. Dominance requires a second copy, an F′ or a specialised transducing phage, making a merozygote. Sex here is not meiosis. It is transfer by conjugation, transformation, or transduction, then recombination into the recipient.
6.2 Conjugation #
6.2.1 F+, F−, Hfr, and F′ #
Lederberg and Tatum recovered prototrophs from a mixture of auxotrophs. Davis showed a filter that let metabolites through stopped the exchange, so contact is required. Free DNA would also have been DNase-sensitive; conjugational DNA is not exposed. F+ cells carry the fertility plasmid and a pilus; F− cells are recipients. Transfer of F usually makes the recipient F+ and moves no chromosomal genes.
An Hfr has F integrated. Transfer starts in F and runs into the chromosome. The bridge almost always breaks before the trailing part of F, so the recipient stays F− and gains only a leading arc. Insertion needs an even number of crossovers: one crossover would open the circle. An F′ is a bad excision that takes neighbouring genes and, once transferred, makes those genes diploid. That merozygote is how bacteria are made to show dominance. Integration and faulty excision are the plasmid version of lambda’s behaviour.
6.2.2 Mapping by time of entry #
Hfr transfer has a fixed origin and direction, so the minute a marker first appears is its position. Wollman and Jacob interrupted mating and scored what had already entered.
The classical E. coli map is 100 minutes and circular, assembled from Hfr strains with different origins and orientations. Final frequency of a late marker is a poor distance, because the bridge breaks. Time of first entry is the map unit. It is minutes, not centimorgans.
6.3 Transformation and transduction #
6.3.1 Transformation #
Transformation is uptake of naked DNA. Griffith saw a heritable change pass from killed smooth pneumococci to live rough cells; Avery, MacLeod, and McCarty showed the substance is DNA. The chemistry is Molecular Biology.
Genetically, two markers are cotransformed when one fragment carries both, so cotransformation falls with distance. Integration again uses an even number of crossovers. No competence recipe is needed to see that the result is short-range linkage.
6.3.2 Generalised and specialised transduction #
A phage particle can inject bacterial DNA. In generalised transduction, P1 (or P22 in Salmonella) occasionally packages a bacterial fragment instead of phage DNA. Any region can move, but two markers are cotransduced only if they share one head, so cotransduction measures short distance. In specialised transduction, lambda excises badly from attB between gal and bio and takes one of those neighbours, leaving some phage genes behind. It cannot move a gene from the far side of the map. Lysogeny and the lambda switch, as regulation, are Molecular Biology. The map consequence is which genes can be specialised-transduced.
| Generalised | Specialised | |
|---|---|---|
| Genes moved | Any, within a headful | Only prophage neighbours |
| Origin of the particle | Mispackaging in the lytic cycle | Aberrant excision |
| Example | P1, P22 | Lambda at attB |
6.4 The cistron #
6.4.1 Complementation #
Benzer defined function with the rII region of phage T4, without needing sequence. Two mutants in one cell, in trans, give a wild-type phenotype if each genome supplies what the other lacks: different cistrons. They fail to complement if they damage the same function. The cis control, both mutations on one genome, should be wild type for ordinary recessive losses. rII has two cistrons, A and B. The same logic uses an F′ or a specialised transducing phage in E. coli. These notes stop at the principle.
Mutations that do not complement can still recombine to a wild-type chromosome, so the cistron is divisible. A point mutation does not recombine to wild type with a deletion that covers it, and can recombine with one that stops short. Mutation and recombination go down to the nucleotide, which Molecular Biology describes. Function, the cistron, is the large unit. A cistron corresponds to one polypeptide.
6.5 Key terms, summary, and self-check #
Key terms. Haploid; merozygote; F plasmid; Hfr; F′; time of entry; cotransformation; generalised transduction; specialised transduction; cistron; cis–trans test.
Summary. Haploidy makes recessive mutations visible at once; a second copy has to be added. F+ donates F, Hfr donates a timed arc of chromosome and usually leaves the recipient F−, and F′ creates a partial diploid. Cotransformation and generalised cotransduction measure short linkage; lambda moves only the neighbours of its attachment site. Complementation in trans defines the cistron, and recombination inside it shows the gene is not an atom.
Self-check.
1. What has to be added to E. coli before dominance can be scored?
2. Why is time of first entry a better Hfr map than the final frequency of a late marker?
3. What did the filter between auxotrophs show about conjugation?
4. Why can lambda move gal and not a gene opposite attB, whereas P1 can move either?
5. Two rII mutations complement and also recombine. Are they one cistron, and what does recombination add?
