A homopolymer stretch composed of variable numbers of cytidine residues inthe terminal repeats of infectious laryngotracheitis virus

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 ampli fy a 750-bp fragment containing this homopolymer stretch. This fragment was chen sequenced directly to determine the number of repeated cytidine resid ues in this homopolymer stretch, which could be used for strain differentia tion. By this procedure, vaccine strains of tissue culture origin could be differentiated into two types: type I contains eight repeated cytidine resi dues, whereas type II contains 10 such residues. Vaccine strains of chicken embryo origin could also be divided into Mo 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 repea ts; the other three field isolates contain 15-19 repeats, which were simila r to the type II chicken-embryo-origin vaccines. The number of repeated cyt idine residues described here could serve as a marker for the strain differ entiation and epidemiologic study of ILTV.