:M4 



chapter 26 



sequently, such experiments prove thai ge- 

 netic recombination occurs between phage 

 panicles in a multiply-injected cell. From 

 the relative frequencies with which differenl 

 recombinants appear among phage released 

 Erom cells multiply-infected with a series of 

 different mutants (this procedure is known 

 as "crossing" genetically-different phages), 

 the genetic map of a phage can be con- 

 structed. When this is done for T4. the 

 mutant loci are found to be arranged in a 

 single closed linear order, that is. a circle 

 (Figure 26-4). 



Because the T4 plaque mutant r lyses 

 rapidly, it produces a larger plaque with 

 sharper margins than the phage with the 

 wild-type allele, r Mixed infections with 

 r and r+ phages usually yield progeny that 

 produce plaques of one or the other type. 

 Two per cent of the observed plaques, how- 

 ever, are mottled; that is, they appear partly 

 r and partly r + . When mottled plaques 

 are picked and their phage content tested, 

 they produce progeny that make either r or 

 r" type of plaque. Since both parental 

 types are present, the mottled plaque could 

 not have been initiated from a single phage 

 haploid for the r locus. Mottled plaques 

 are not caused by infection with clumps of 

 phage particles; moreover, these plaques are 

 not initiated by a phage carrying an un- 

 stable r mutant, since unstable r phages pro- 

 duce phenotypically — and genotypically — 

 sectored, not mottled, plaques. From these 

 results and others, it has been proved 6 that 

 the two per cent of T4 phage producing 

 mottled plaques in mixed infections are het- 

 erozygous for a short region of the phage 

 genome that includes the r locus. 



A single phage particle can be hetero- 

 zygous for several loci, provided they are 



GENOTYPE 



rl or rill Mutants 



rll Mutants 

 -+ 



PLAQUES FORMED 

 ON HOST STRAIN 



B K 



r r 



r None 



+ + 



i k.i RE 26-5. Behavior of r mutants of T-even 

 phages in the B and K strains of E. coli. 



located far enough apart. In fact, it is un- 

 likely that any phage is completely haploid. 

 Regions present in diploid condition are 

 said to be redundant. Redundancy appears 

 to be accomplished in two ways 7 : Either 

 both regions are part of one DNA double- 

 helix {terminal redundancy) or the extra re- 

 gion is present as a separate segment of 

 double-helix DNA {internal redundancy) . 



Genetic Fine Structure of <£T4 B 



The r mutants occur in three distinct re- 

 gions of the T4 genetic map — rl, rll, and 

 rill. The r mutants in all three regions 

 produce plaques when E. coli strain B is 

 used as host. However, mutants in the rll 

 region are unique in that they cannot form 

 plaques when their host is strain K.12 of 

 E. coli (which happens to be lysogenic for 

 lambda), whereas the rl and /•/// mutants 

 and r+ phages can (Figure 26-5). Thus, 

 among r mutants, only those in region II 

 have this restriction in host range. 



The host-range restricted rll mutant is 

 useful since it can be employed as a selec- 

 tive marker. The mutation frequency from 



After the work of M. Delbruck and W. T. Bai- 

 ley, and of A. D. Hershey and R. Rotman. 

 '•See A. D. Hershey and M. Chase (1951); see 

 also A. H. Doermann and L. Boehner (1963). 



7 See G. Streisinger, R. S. Edgar, and G. H. Den- 

 hardt (1964). 



s The following discussion is based mainly upon 

 the work of S. Benzer (1955, 1957). 



