ABSTRACTThe nonpathogenic human adeno-associated virus type 2 (AAV-2) has adopted a unique mechanism to site-specifically integrate its genome into the humanMBS85gene, which is embedded inAAVS1on chromosome 19. The fact that AAV has evolved to integrate into this ubiquitously transcribed region and that the chromosomal motifs required for integration are located a few nucleotides upstream of the translation initiation start codon ofMBS85suggests that the transcriptional activity ofMBS85might influence site-specific integration and thus might be involved in the evolution of this mechanism. In order to begin addressing this question, we initiated the characterization of the humanMBS85promoter region and compared its transcriptional activity to that of the AAV-2 p5 promoter. Our results clearly indicate thatAAVS1is defined by a complex transcriptional environment and that theMBS85promoter shares key regulatory elements with the viral p5 promoter. Furthermore, we provide evidence for bidirectionalMBS85promoter activity and demonstrate that the minimal motifs required for AAV site-specific integration are present in the 5′ untranslated region of the gene and play a posttranscriptional role in the regulation ofMBS85expression. These findings should provide a framework to further elucidate the complex interactions between the virus and its cellular host in this unique pathway to latency.