The CAEV tat gene trans-activates the viral LTR and is necessary for efficient viral replication

Caprine arthritis-encephalitis virus (CAEV) is a lentivirus which is closely related by nucleotide sequence and biological properties to visna virus. Sequence analysis of the CAEV genome revealed the presence of a small open reading frame (ORF) which shares amino acid identity with the visna virus tat gene. Using an infectious molecular clone of CAEV the role of the tat ORF in viral replication was examined. Mutations were made in the tat ORF that introduced two in frame stop codons six amino acids downstream of the tat AUG; in addition, a deletion mutant was made that removed most of the tat ORF. Both of these mutants had greatly reduced virus titers (> 1000-fold less than the wild type infectious clone). Co-transfection of a tat expressing plasmid with these viruses containing the tat ORF mutations resulted in higher levels of virus production demonstrating that the effects of both mutants are tat specific. These mutants provide data that the CAEV tat gene is necessary for efficient virus replication. Analysis of the RNA in these transfected cells showed that complementation of the tat gene was in trans and not the result of recombination. Analysis of the gag and rev proteins in the transfected cells demonstrated that these proteins were not detectable in cells transfected with the tat mutants but could be readily detected when the mutations were complemented in trans with a tat expression vector. To test for tat mediated trans-activation a plasmid expressing the CAEV tat ORF was co-transfected with plasmids containing either the CAEV or visna virus LTR driving transcription of the bacterial chloramphenicol acetyltransferase gene (CAT). These experiments indicate that one function of the CAEV tat protein is to trans-activate gene expression from the viral promoter. RNase protection analysis of CAT mRNA from co-transfected cells demonstrated that CAEV Tat trans-activates gene expression by increasing steady-state levels of mRNA.