GENOME PLASTICITY IN THE DISTAL TAIL FIBER LOCUS OF THE T-EVEN BACTERIOPHAGE - RECOMBINATION BETWEEN CONSERVED MOTIFS SWAPS ADHESIN SPECIFICITY

The adsorption specificity of the T-even phages is determined by the p rotein sequence near the tip of the long tail fibers. These adhesin se quences are highly variable in both their sequence and specificity for bacterial receptors. The tail fiber adhesin domains are located in di fferent genes in closely related phages of the T-even type. In phage T 4, the adhesin sequence is encoded by the C-terminal domain of the lar ge tail fiber gene (gene 37), but in T2, the adhesin is a separate gen e product (gene 38) that binds to the tip of T2 tail fibers. Analysis of phage T6 and Ac3 sequences reveals additional variant forms of this locus. The tail fiber host specificity determinants can be exchanged, although the different loci have only limited homology. Chimeric fibe rs can be created by crossovers either between small homologies within the structural part of the fiber gene or in conserved motifs of the a dhesin domain. For example, the T2 adhesin determinants are flanked by G-rich DNA motifs and exchanges involving these sequences can replace the specificity determinants. These features of the distal tail fiber loci genetically link their different forms and can mediate acquisiti on of diverse host range determinants, including those that allow it t o cross species boundaries and infect taxonomically distant hosts. (C) 1998 Academic Press.