#bacterial gene transfer

Microbial genetics

Interrupted mating experiment maps genes based on

Wollman and Jacob designed interrupted mating to order genes by time they enter recipient after mixing Hfr donor and F- recipient. At intervals, conjugation pairs are disrupted by vigorous blending in Waring blender, stopping DNA transfer. Recipients plated for early, middle, and late markers reveal sequential appearance. Gene entering first lies closest to oriT, next gene transfers seconds to minutes later depending on distance along chromosome. Plotting time of entry versus marker yields circular genetic map. This temporal mapping revolutionized E. coli linkage analysis before DNA sequencing.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Interrupted Mating Experiment; Hartl, Chapter 6

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Microbial genetics

Specialized transduction transfers

Specialized transduction occurs only with temperate phages that integrate at specific attachment site forming prophage. Upon induction, excision normally precise, but rare imprecise excision deletes part of prophage and captures adjacent bacterial genes, such as lambda gal and bio near attB in E. coli. Resulting defective phage carries bacterial genes but lacks essential phage genes, requiring helper phage for propagation. Only genes flanking integration site transfer, at high frequency for those markers, not random genome. This specificity contrasts generalized transduction and explains lambda transducing phages lambda dgal and lambda dbio.

Ref: Nature Scitable, Specialized Transduction; Hartl, Chapter 6: Lysogenic Phage and Adjacent Gene Transfer

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Microbial genetics

Generalized transduction transfers

Generalized transduction results from packaging error during lytic assembly of temperate or virulent phage such as P1 or P22. Phage terminase occasionally recognizes pseudo-pac site on host chromosome and fills capsid with random host fragment of size limited by head capacity, about 100 kb for P1. Any bacterial gene can be packaged with roughly equal probability, so lysate contains mixture covering entire genome. Transducing particles infect new cells but inject only bacterial DNA, which recombines. This randomness allows mapping any gene and building linkage clusters via co-transduction analysis.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Generalized Transduction Mechanism

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Microbial genetics

Co-transformation is useful for

Co-transformation detects simultaneous uptake and integration of two markers from same donor fragment, indicating they reside within length of transforming DNA. By scoring frequency of double transformants versus single, geneticists estimate proximity and linear order of three factors using two-point and three-point crosses. Higher co-transformation implies shorter separation. Beadle-Tatum era used auxotrophic markers to construct first bacterial maps before conjugation mapping. Because transformation does not require cell contact, it allows mapping closely linked genes that interrupted mating cannot resolve, resolving clusters like tryptophan operon organization.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 6: Gene Mapping by Cotransformation; NCBI Bookshelf

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