Irwin Tessman

Some unusual properties of the nucleic acid in bacteriophages S13 and ∅X174

Abstract Phages ∅X174 (and S13), labeled with P 32 , are inactivated by decay of their incorporated radioactive atoms with approximately 100% efficiency, even at −196° C (assuming the DNA content of ∅X174 to be as determined by Sinsheimer). The high sensitivity to P 32 decay suggests an unusual structure and function of the DNA in these phages. One possibility is a single-stranded DNA. The efficiency of inactivation of ∅X174 is measured at temperatures up to 37°. The rate of death of the phage at the elevated temperatures is too great to be entirely due to decay of the incorporated P 32 atoms. This suggests a technical difficulty in studying the effects of temperature on the lethality of phage-incorporated P 32 atoms. The resemblance of S13 and ∅X174 to some plant and animal viruses is discussed.

Multiplicity reactivation of bacteriophage T1

Abstract Multiplicity reactivation of ultraviolet-inactivated T1 can be detected using suitably irradiated bacteria as host cells. Under certain conditions the amount of multiplicity reactivation is independent of the dose on the bacteria, even when the dose is reduced to zero so that normal cells are infected. The results are consistent with the idea that in normal cells some other reactivation phenomenon masks multiplicity reactivation of T1.

The relative radiosensitivity of bacteriophages S13 and T2.

Abstract P 32 -labeled S13 bacteriophages are inactivated by decay of their incorporated radioactive atoms at 1 7.8 the rate of death of equally radioactive T2 bacteriophages. The ratio of the sensitivities to X-rays of these two bacteriophage strains is the same as that of their sensitivities to P 32 decay. If it is assumed that in S13, as in T2 and other bacteriophage strains previously studied, only one out of every ten P 32 disintegrations kills the phage particle in which it occurs, one may estimate on the basis of these results that S13 contains 3.0 × 10 −17 g of nucleic acid per particle. It is possible, however, that S13 is intrinsically much more radiosensitive than other bacteriophages, and a lower limit of the nucleic acid content of 3.0 × 10 −18 g per S13 particle can be estimated and would obtain if every P 32 disintegration is lethal.

Potential for variability through multiple gene products of bacteriophage ΦX174

The small single-stranded DNA phages ΦX174 and S13 produce multiple products of certain phage genes, as observed by electrophoresis on SDS–polyacrylamide slab gels. Two A protein products, two A* products and four G products are observed. The multiple gene products may arise from multiple sites for initiation or termination of translation, or by protein modification. Some of the variant products may provide a substitute for heterozygosity without a concomitant increase in the size of the genome.

The Genes of the Isometric Phages and Their Functions

The small isometric DNA phages ϕ X174 and S13 are currently believed to encode nine genes, which specify at least 16 gene products (Fig. 1). The concept that each gene on a DNA molecule is physically distinct from every other gene has been shattered for this class of phages. The production of two proteins from one DNA sequence was reported by Linney et al. (1972), who found that translation of the gene- A region was initiated at two different sites in the same reading frame. The spectacular finding by Barrell et al. (1976), Sanger et al. (1977), and Weisbeek et al. (1977) of overlapping sequences that are translated in different reading frames, yielding functional proteins, provided an entirely new view of gene topology. By the criteria of complementation tests, which measure units of genetic function, there are at least eight genes in these phages (Jeng et al. 1970), designated A through H . To these must be added gene J , which specifies a major capsid component but is not yet represented by any known mutation. The “one gene-one protein” hypothesis can be converted into a definition of a gene and used as a biochemical method of counting genes. Any stretch of DNA that codes for a protein is counted as one gene, whether or not some other protein is coded for somewhere in that same stretch of DNA. This method of identifying genes is implicit in current research. We are faced with a plethora of gene products, only some of whose functions have...