5'-terminal deletions are a common feature of endogenous retrovirus loci located on chromosome 1 of White Leghorn chickens.

My previous studies demonstrated that chromosome 1 has all five of the endogenous retrovirus loci associated with nonexpression of viral proteins (gs- chf-) in White Leghorn chickens. The current study was done to localize the two defective endogenous retrovirus loci, ev3 and ev6, to determine whether nonexpression of the viral loci on chromosome 1 is a transcriptional prerequisite or a result of an underlying structural defect. The structure of ev6 is every similar to that of two other gs- chf- -associated loci, ev4 and ev5; all three contain a 5'-terminal deletion that eliminates the viral transcriptional promoter. Unlike the gs- chf- -associated loci, ev6 apparently uses an adjacent cell promoter to produce high levels of viral glycoprotein. The ev6 locus was located on the long arm of chromosome 1 by in situ hybridization, thus indicating that transcription of an endogenous retrovirus locus on chromosome 1 can occur. However, the 5'-terminal deletion in several viral loci on chromosome 1, but not on other chromosomes, suggests that the deletion may result from the events that generate or maintain these loci in the White Leghorn population. The additional finding of ev3, which has an internal deletion, on a microchromosome suggests that the mechanism that produced deletions in many chromosome 1 loci is specific for the 5' terminus.