Questions: Generalized Transduction and Phage-Mediated Gene Transfer

Question 1

What makes a phage-mediated gene transfer event 'generalized' rather than 'specialized' transduction?

• A. The phage can infect a broad range of bacterial species, transferring genes across species barriers
• B. The packaging machinery accidentally incorporates random bacterial DNA fragments during lytic replication, so any gene has roughly equal probability of being transferred
• C. The transferred DNA always integrates at a specific chromosomal locus in the recipient cell
• D. The process uses general recombination factors that are present in all bacterial cells

Answer: (B)

Generalized transduction is called 'generalized' because the error that creates transducing particles — the packaging machinery picking up random bacterial DNA fragments — is indiscriminate. Any part of the degraded bacterial chromosome can be accidentally packaged, so all genes have roughly equal probability of being transferred. This is in contrast to specialized transduction, where only genes adjacent to a specific phage integration site are transferred. The 'generalized' label refers to which genes can be transferred, not the host range of the phage.

Question 2

A transducing phage particle infects a new bacterium and injects its contents. Compared to a normal phage infection, what most likely happens next?

• A. A lytic cycle begins, producing new transducing phage that carry only bacterial DNA
• B. No infection can proceed because the particle's surface proteins are incompatible with the new host's receptors
• C. The injected bacterial DNA may undergo homologous recombination with the recipient's chromosome; no lytic cycle begins because the particle carries no phage genes
• D. The particle injects DNA and initiates lysogeny, integrating the bacterial genes at a phage attachment site

Answer: (C)

A transducing particle looks like a normal phage on the outside (same capsid and tail proteins) so it can attach and inject its contents normally. But it carries bacterial DNA, not phage DNA — so no phage genes are introduced and no lytic or lysogenic cycle can begin. The injected bacterial fragment can survive and, if it shares sufficient sequence homology with the recipient's chromosome, undergo homologous recombination to stably integrate. If recombination fails, the DNA is eventually degraded.

Question 3

Because the phage packaging mechanism operates by randomly sampling bacterial DNA fragments, any gene on the donor bacterium's chromosome can in principle be transferred to a recipient by generalized transduction.

• T. True
• F. False

Answer: True

This randomness is the defining feature of generalized transduction. The packaging machinery (terminase) does not verify it is packaging phage DNA — it simply fills a capsid with a headful of DNA. Any bacterial chromosome fragment of approximately the right size can be picked up. In practice, transfer frequency depends on the physical proximity of genes to packaging start sites, but no gene is categorically excluded from transduction.

Question 4

Transducing phage particles typically account for a large fraction of the phage released from an infected cell, making generalized transduction a highly efficient mechanism for horizontal gene transfer in nature.

• T. True
• F. False

Answer: False

Transducing particles are quite rare — roughly 1 in 10⁶ to 10⁸ phage particles is a transducing particle. The packaging machinery correctly packages phage DNA in the vast majority of events. The frequency is low per infection event, but given the astronomical number of phage infections occurring in natural environments (estimated at 10²³ infections per second globally), even a low per-event rate makes generalized transduction a significant evolutionary force at the population level.

Question 5

Describe the specific error in phage packaging that creates a transducing particle, and explain what determines whether the transferred bacterial DNA is permanently inherited by the recipient bacterium.

Model answer: The packaging error occurs when the terminase (packaging enzyme), which normally recognizes and fills a phage capsid with phage DNA, instead picks up a fragment of the bacterial chromosome. The enzyme does not verify the DNA's identity — it simply fills the head with a 'headful' of DNA from whatever is available. The resulting transducing particle contains bacterial DNA in a normal phage protein shell. Whether the transferred DNA is permanently inherited depends on whether it undergoes successful homologous recombination with the recipient's chromosome; if homologous sequences are present and recombination occurs, the donor genes are stably integrated. If recombination does not occur, the fragment is eventually degraded.

The packaging error is the key mechanistic insight: the terminase measures size (a headful), not identity. The outcome in the recipient is governed by classical homologous recombination machinery — the recipient needs regions of sequence similarity flanking the genes of interest. This means the most reliably transferable genes are those for which recipient strains carry related sequences, which in practice includes genes encoding antibiotic resistance, metabolic enzymes, and other horizontally mobile elements.