Overlapping Genes

The first viral genome to be completely sequenced, that of bacteriophage ^X174, revealed surprising information: the nucleotide sequences of several genes overlapped. This genome encodes nine proteins ( FIGURE 8.33). Two of the genes are nested within other genes; in both cases, the same DNA sequence codes for two different proteins by using different reading frames (see p. 000 in Chapter 13). In five of the ^X174 genes, the initiation codon of one gene overlaps the termination codon of another.

The results of subsequent studies revealed that overlapping genes are found in a number of viruses and bacteria. Viral genome size is strictly limited by the capacity of the viral protein coat; so there is strong selective pressure for economic use of the DNA. __

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