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The Genes of the Isometric Phages and Their Functions

Ethel S. Tessman, Irwin Tessman


The small isometric DNA phages ϕX174 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...

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