A homopolymer stretch composed of variable numbers of cytidine residues in the terminal repeats of infectious laryngotracheitis virus.

ABSTRACT

A homopolymer stretch composed of variable numbers of cytidine residues was found within the inverted terminal repeats of infectious laryngotracheitis virus (ILTV). A polymerase chain reaction procedure was developed to amplify a 750-bp fragment containing this homopolymer stretch. This fragment was then sequenced directly to determine the number of repeated cytidine residues in this homopolymer stretch, which could be used for strain differentiation. By this procedure, vaccine strains of tissue culture origin could be differentiated into two types type I contains eight repeated cytidine residues, whereas type II contains 10 such residues. Vaccine strains of chicken embryo origin could also be divided into two types type I contains mainly 11 repeated cytidine residues, whereas type II contains 15-21 such repeats. In comparison, two of the five field isolates examined contain 12-13 repeats; the other three field isolates contain 15-19 repeats, which were similar to the type II chicken-embryo-origin vaccines. The number of repeated cytidine residues described here could serve as a marker for the strain differentiation and epidemiologic study of ILTV.