In several herpesviruses the genes for the major DNA binding protein (MDBP), a putative assembly protein, the
glycoprotein B (gB), and the viral DNA polymerase (pol) coliocate. In murine cytomegalovirus (MCMV), two members
of this gene block, pol (Elliott, Clark, Jaquish, and Spector, 1991, Virology 185, 169-186) and gB (Rapp, Messerle,
BOhler, Tannheimer, Keil, and Koszinowski, 1992, J. Virol., 66,4399-4406) have been characterized. Here the two other
MCMV genes are characterized, the gene encoding the MDBP and the ICP18.5 homolog encoding a putative
assembly protein. Like in human cytomegalovirus (HCMV) the genes order is pol, gB, ICP18.5, and MDBP. The 4.2-kb
MDBP mRNA is expressed first in the early phase, whereas the 3.0-kb ICP18.5 mRNA is a late transcript. The open
reading frame of the MDBP gene has the capacity of encoding a protein of 1191 amino acids with a predicted molecular
mass of 131.7 kDa. The MCMV ICP18.5 ORF is translated into a polypeptide of 798 amino acids with a calculated
molecular mass of 89.1 kDa. Comparison of the amino acid sequences of the predicted proteins of MCMV with the
respective proteins of HCMV, Epstein-Barr virus (EBV), and herpes simplex virus type-1 (HSV-1) reveals a striking
homology ranging from 72% (HCMV), 50% (EBV), to 45% (HSV-1) for the MDBP sequence and from 74% (HCMV), 51 %
(EBV), to 49% (HSV-1) for the ICP18.5 sequence. These results establish the elose relationship of the two cytomegaloviruses,
and underline the usefulness of the murine model for studies on the biology of the CMV infection
