Bacteriophage ΦlX174 : Viral Genes and Functions

Citation

Hutchison, Clyde Allen-III (1969) Bacteriophage ΦlX174 : Viral Genes and Functions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/82XF-B518. https://resolver.caltech.edu/CaltechETD:etd-08092006-162535

Abstract

Mutants. Conditional lethal mutants of bacteriophage [phi]X174 ([phi]X) have been selected. These include nonsense mutants of the am (amber nonsense triplet - UAG)and op (opal nonsense triplet - UGA) types. Mis-sense mutants of the ts (temperature-sensitive) and ss (solvent-sensitive) types have also been isolated. The ss mutants are abnormally sensitive, during plaque formation, to the presence of solvents such as dimethylsulfoxide, ethylene glycol, or urea in the growth medium. Host range mutants are also described. Double and triple mutant strains have been constructed from specific single mutants.

Cistrons. Complementation studies (principally with am and ts mutants) have defined seven complementation groups or cistrons. It is believed that these represent the genes for seven essential [phi]X-coded proteins. At least six of these cistrons are homologous, by complementation, to cistrons of the related phage S13.

Functions. Studies of abortive infection by conditional lethal mutants under restrictive conditions have been performed in order to elucidate the functions of the mutant cistrons. Properties of phage particles produced by mutants have been examined in order to identify cistrons which code for components of the phage coat. The cistron functions deduced from these studies are: cistron I - lysis of the host. Mutants in this cistron produce progeny at a normal rate, and for an extended time, but are not able to lyse the host cell. cistron II - spike component. This cistron codes for a protein which is a component of the spikes which project from the 12 vertices of the isometric [phi]X capsid. cistron III - spike component. This cistron codes for a second protein component of the phage spike and contains the serum-blocking antigen of the phage particle. cistron IV - phage coat. This cistron codes for a protein of the phage coat which is probably, by elimination, the main structural component of the capsid. cistron V - ?. The function of this cistron product is not known. cistron VI - RF replication. The product of this cistron is necessary for replication of the double-stranded form of [phi]X-DNA (RF). cistron VII - spike component. The product of this cistron is believed to be a component of the phage spike which determines the host range of the particle.

These physiological studies suggest that assembly of phage, or defective phage particles is necessary for the synthesis of viral single-stranded DNA. RF replication (which requires cistron VI function and cistron V function are also necessary for single-strand synthesis.

Recombination experiments (involving both two and three factor crosses) indicate that the order of the cistrons is II-III-VII-V-I-IV-VI, possibly joined end to end to form a circular map. It is of interest that cistrons which code for the spike components (II, III, and VII) appear to be contiguous on the map. A polar am mutant in cistron IV is deficient also in cistron VI function. This is interpreted to mean that reading proceeds from left to right on the map as shown (5' to 3' direction of the viral DNA and [phi]X mRNA).

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