GENOMIC POLYMORPHISM IN THE T-EVEN BACTERIOPHAGES

We have compared the genomes of 49 bacteriophages related to T4. PCR a nalysis of six chromosomal regions reveals two types of local sequence variation. In four loci, we found only two alternative configurations in all the genomes that could be analyzed. In contrast, two highly po lymorphic loci exhibit variations in the number, the order and the ide ntity of the sequences present. In phage T4, both highly polymorphic l oci encode internal proteins (IPs) that are encapsidated in the phage particle and injected with the viral DNA. Among the various T4-related phages, 10 different ORFs have been identified in the IP loci; their amino acid sequences have the characteristics of internal proteins. At the beginning of each of these coding sequences is a highly conserved 11 amino acid leader motif. In addition, both 5' and 3' to most of th ese ORFs, there is a similar to 70 bp sequence that contains a T4 earl y promoter sequence with an overlapping inversely repeated sequence. T he homologies within these flanking sequences may mediate the recombin ational shuffling of the IP sequences within the locus. A role for the new IP-like sequences in determining the phage host range is proposed since such a role has been previously demonstrated for the IP1 gene o f T4.