Microbial genetics

Microbial genetics

In bacteriophage T4, intragenic recombination was studied by

Seymour Benzer studied intragenic recombination within rII locus of bacteriophage T4, demonstrating gene is divisible linear array rather than indivisible bead. Using overlapping deletions of rIIA and rIIB cistrons, he crossed thousands of mutants, scoring wild-type recombinants on restrictive K12(lambda) host unable to support rII mutant growth. Recombination frequency between adjacent sites measured subgenic distances, building fine-structure map with resolution down to single base pair. Work introduced terms cistron, muton, recon and proved collinearity of recombination map and coding sequence.

Ref: Benzer S, PNAS 1955-1961, Fine Structure of rII Region; Griffiths et al., Chapter 9: Intragenic Mapping in T4

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Microbial genetics

Co-transduction frequency is inversely proportional to

Co-transduction scoring after P1 or P22 generalized transduction measures how often two bacterial markers transfer together in same phage head. Packaging capacity limits fragment size; genes within same head length co-occur frequently. If distance between genes exceeds headful, they cannot fit together, co-transduction drops to zero. Intermediate separations show recombination between them inside recipient reducing co-inheritance. Therefore frequency is inversely proportional to map distance: very close genes show near 100% co-transduction, farther genes show lower percentages, enabling fine-structure mapping and ordering using Wu formula d = [1 - (cofreq)^(1/3)] × length.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Cotransduction Frequency and Distance

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Microbial genetics

Cells containing F′ plasmid are

Cells containing F' plasmid bear normal haploid chromosome plus extra copy of genes carried on F', creating merodiploid – partial diploid state where those loci are duplicated while rest genome haploid. Term mero means partial, applied by Jacob and Wollman. Merodiploids allow complementation and dominance tests for chromosomal alleles without constructing true diploids impossible in bacteria. For example F' lac+ / lac- mero expresses beta-galactosidase, showing lac+ dominant. Unstable segregation can lose F', reverting to haploid, useful for allelic exchange analysis.

Ref: Hartl & Jones, Genetics, 8th ed., Chapter 6: Merodiploidy and F Prime Genetics; NCBI Bookshelf

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Microbial genetics

F′ plasmid formation occurs due to

F' plasmid formation results from imprecise excision of integrated F factor in Hfr chromosome. F integration involves homologous recombination between plasmid and chromosome IS sequences. When excising, recombination occurs between homologous sites but at incorrect boundaries, looping out plasmid that carries adjacent chromosomal segment flanking oriT. Resulting F' retains tra genes plus chromosomal genes like lac, making larger hybrid plasmid. Upon transfer, recipient becomes partial diploid for those genes. Correct excision regenerates normal F+ and leaves chromosome intact without extra bacterial genes.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: F Prime Formation Mechanism

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Microbial genetics

Gene distances in interrupted mating are measured in

In interrupted mating, transfer rate of chromosomal DNA via conjugative pilus is approximately constant, roughly 45 kilobases per minute under standard conditions at 37°C for HfrH, so physical distance translates to temporal separation. Therefore map length is expressed as minutes needed for marker to enter recipient after mating starts, representing minutes of transfer time. Whole E. coli chromosome maps as about 100 minutes. Different Hfr strains with origins at distinct sites generate overlapping time maps, allowing assembly of complete circular linkage map and orientation determination from entry order.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 6: Map Units in Minutes; Wollman & Jacob 1955

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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

Hfr strain differs from F+ strain because

F+ strains carry autonomous F plasmid extrachromosomally, expressing pili and transferring only F DNA in crosses, donor ability but chromosomal genes transfer at very low frequency. Hfr strains arise when F integrates into bacterial chromosome via homologous recombination at IS elements, creating single linkage between F origin oriT and chromosome. Upon conjugation, transfer initiates at integrated oriT and proceeds linearly into chromosomal DNA, transferring chromosomal markers with high efficiency in order determined by integration site and orientation, converting recipients to recombinants for those alleles while remaining F- unless whole chromosome transfers.

Ref: Wollman & Jacob 1956, Comptes Rendus; Griffiths et al., Chapter 6: Hfr vs F+ Biology

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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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Microbial genetics

Genes close together show high frequency of

During natural transformation, competence apparatus imports fragments of extracellular DNA typically tens of kilobases long. If two genes lie close on same DNA piece, they enter same recipient together rather than independently. Close linkage shows high co-transformation frequency because single incoming molecule carries both alleles. As intergenic distance increases, probability molecule breaks between them rises, reducing co-inheritance. Co-transformation frequency thus inversely measures physical distance, analogous to cotransduction. Mapping exploits this to order genes and estimate spacing in bacterial chromosomes without conventional meiotic recombination.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Transformation Mapping and Linkage

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Microbial genetics

Transforming principle identified by Avery was

Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944 purified transforming principle from heat-killed S pneumococci. They sequentially removed lipids, polysaccharides, and proteins with enzymes, showing activity remained after protease and RNase treatments but was abolished by deoxyribonuclease. Chemical analysis revealed DNA with high viscosity, phosphorus content, and diphenylamine reaction positive. Transformation of rough to smooth required intact DNA, proving DNA rather than protein carries genetic specificity. This milestone established DNA as genetic material preceding Hershey-Chase confirmation and transformed molecular biology understanding.

Ref: Avery et al., Journal of Experimental Medicine 1944: Studies on Chemical Nature of Transforming Principle; Hartl, Chapter 1

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