The IE2 region of the human cytomegalovirus (CMV) strain Towne major immediate-early (MIE) gene encodes a transcriptional transactivator that stimulates expression from a variety of heterologous target promoters but specifically down-regulates its own promoter. By immunofluorescence and Western immunoblot analysis with monospecific peptide antisera, we found that human CMV MIE exon 5 encodes four overlapping polypeptides, two present at immediate-early times (80 and 55 kDa) and two others detected only at late times after infection (55 and 40 kDa). However, only the 80-kDa version (579 amino acids), which is derived from the small upstream exons 2 and 3 fused to the intact exon 5 region, was functionally active in both transactivation and autoregulation as assessed by cotransfection experiments. These results confirm the corrected assignment of the coding capacity of the exon 5 region based on amino acid homology with the equivalent IE2 protein from simian CMV (Colburn). In transient DNA transfection assays, IE2 expression plasmids also produced a predominant full-length 80-kDa protein, which was localized in a distinctive reticular pattern in the nucleus. Two short basic nuclear localization signals in IE2 were identified by deletion analysis and by conversion of a test cytoplasmic herpes simplex virus protein into a form that localized in the nucleus after insertion of either of these two human CMV motifs. Functional assays with MIE region plasmids containing deletions or truncations in exon 5 revealed that both transactivation and autoregulation required several distinct domains within the COOH half of the IE2 protein, whereas a region between codons 99 and 194 could be discarded. Three segments at the NH2 end of the protein between codons 1 to 85, 86 to 98, and 195 to 290 were also essential for transactivation but played no role in autoregulation. Finally, in domain swap experiments, GAL4-fusion proteins containing either an NH2-terminal 51-amino-acid domain from exon 3 (codons 25 to 85) or the COOH-terminal 33-amino-acid domain from exon 5 (codons 544 to 579) identified two distinct activator domains from IE2, both of which have acidic characteristics.