Questions: Specialized Transduction and Prophage Excision

Question 1

A student claims that phage lambda can transduce any bacterial gene, given enough rounds of transduction — just with very low efficiency for distant genes. What is wrong with this claim?

• A. Nothing — phage lambda can transduce any gene, like all transducing phages, just at different efficiencies.
• B. Lambda can only transduce genes immediately flanking its specific chromosomal att site (the gal and bio operons); genes elsewhere are inaccessible to specialized transduction regardless of how many events occur.
• C. Lambda cannot transduce at all — specialized transduction refers to a different type of phage.
• D. Lambda can transduce genes up to a few kilobases from the att site, but efficiency falls sharply beyond that distance.

Answer: (B)

This is the defining feature of specialized transduction. Imprecise excision can only grab sequences flanking the integration site — there is no mechanism by which sequences on the other side of the chromosome could be incorporated. For phage lambda, the att site sits between the gal and bio operons, so only gal and bio genes can be transduced, always and only. This hard spatial boundary distinguishes specialized transduction from generalized transduction (where headful packaging can accidentally incorporate any chromosomal fragment).

Question 2

What makes a specialized transducing phage particle 'defective'?

• A. It carries a mutated phage genome that cannot replicate due to accumulated point mutations.
• B. It carries some bacterial DNA in place of phage DNA that was left behind during imprecise excision, and therefore cannot complete a lytic cycle without complementation.
• C. The bacterial DNA it carries is too large to fit with the full phage genome, so phage packaging is impaired.
• D. The bacterial DNA it carries disrupts expression of phage structural genes through antisense interference.

Answer: (B)

Imprecise excision is asymmetric: the recombination machinery cuts at the wrong position, incorporating adjacent bacterial DNA on one side and simultaneously leaving behind a corresponding stretch of phage DNA in the bacterial chromosome. The resulting particle has a hybrid genome — part phage, part bacterial — but is missing essential phage genes. It can inject its DNA into a new host cell, where the bacterial genes can recombine into the chromosome, but it cannot complete a lytic cycle on its own. Complementation by a co-infecting wild-type phage can rescue lytic replication.

Question 3

In specialized transduction, the same flanking bacterial genes are delivered to recipient cells every time a defective transducing phage infects, making this form of gene transfer highly efficient for those specific genes.

• T. True
• F. False

Answer: True

Because the defective phage always carries the same bacterial DNA — determined by what flanked the att site at the time of imprecise excision — every transduction event delivers identical genetic cargo. This predictability and repeatability makes specialized transduction far more efficient than generalized transduction for any specific gene near the att site. It was this reliability that made specialized transduction a powerful tool for early fine-structure genetic mapping near phage integration sites, before modern molecular cloning techniques were available.

Question 4

Specialized transduction requires that the prophage excises precisely and mostly from the bacterial chromosome, regenerating the original integration boundaries.

• T. True
• F. False

Answer: False

Specialized transduction results from precisely the opposite of precise excision. Normal (precise) excision is the exact reversal of integration — recombination between attL and attR regenerates intact phage and bacterial chromosomes with no gene exchange. Specialized transduction requires *imprecise* excision, where the recombination machinery cuts asymmetrically and at the wrong position, incorporating flanking bacterial DNA into the phage genome while leaving phage DNA behind. This happens rarely — roughly 10⁻⁵ to 10⁻⁶ of excision events — making it an infrequent but consequential mistake.

Question 5

What distinguishes specialized transduction from generalized transduction in terms of which bacterial genes can be transferred, and what mechanistic difference explains this?

Model answer: In generalized transduction, any bacterial gene can be transferred because phages like P1 use a 'headful' packaging mechanism that occasionally incorporates random chromosomal fragments instead of phage DNA — the process has no sequence specificity. In specialized transduction, only genes immediately flanking the prophage's chromosomal att site can be transferred, because the mechanism requires imprecise excision of the integrated prophage: the recombination machinery must include adjacent bacterial DNA, which is spatially constrained to sequences flanking the integration site. Genes elsewhere on the chromosome are inaccessible regardless of how many transduction cycles occur.

The mechanistic contrast is the key: generalized transduction is a packaging accident (random fragments get packaged), while specialized transduction is an excision accident (the prophage grabs neighboring chromosomal sequence). The spatial restriction in specialized transduction is a direct consequence of integration site specificity — the phage integrates at a defined location and can only 'steal' what's nearby.