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Holak, Karina (2007): Investigation of murine cytomegalovirus US22 gene family members m139 and m142. Dissertation, LMU München: Faculty of Medicine
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Abstract

Human cytomegalovirus is a ubiquitous human pathogen, causing disease in the immunocompromised host. Most of its ORFs have not been well studied due to a limited host range and slow growth of HCMV in cultured cells. MCMV, a natural pathogen isolated from mice, constitutes the most amenable animal model for human β-herpesviruses. To date most of its approximately 200 genes have an unknown function. For the analysis of these genes straightforward mutagenesis methods are necessary. With the cloning of herpesviruses as an infectious bacterial artificial chromosome a novel approach of mutagenesis has been established. Herpesviruses are now accessible to the tools of bacterial genetics. Since then site-directed mutagenesis by homologous recombination using linear DNA fragments and random transposon BAC mutagenesis have been introduced to delineate the functions of viral ORFs. The purpose of this work was to analyze two members of the US 22 gene homolog family, genes m139 and m142, with site-directed mutagenesis. Members of this family are conserved in all herpesviruses and mostly have unknown functions. Transposon mutants showed a macrophage phenotype for m139, whereas m142 was possibly essential for viral replication. Genes m139-m141 and m142-m143 have complex transcriptional regions and have 3´-coterminal transcripts. The insertion of a 3-kb large transposon could destabilize the upstream transcripts. Site-directed mutants of genes m139 and m142, where only the start codon is deleted, should not influence transcript stability and permit confirmation of the results obtained with transposon mutagenesis. Targeted mutants of MCMV BAC were constructed for ORF m139 (ΔATG-m139) and m142 (ΔATG-m142, ΔATG-m142/FRT) by homologous recombination using linear DNA fragments. Mutant ΔATG-m139 showed attenuated growth in peritoneal macrophages. This mutant, with the first two ATGs deleted, expressed a truncated protein of 61 kDa. Gene m139 seems to act in cooperation with genes m140 and m141 on the protein level. The site-directed MCMV BAC mutant of ORF m142 on the other hand could not reconstitute viral progeny in eukaryotic cells. The ORF of m142 was inserted an ectopic position and viral progeny was reconstituted with this revertant. Thus, it was shown that gene m142 is essential for viral replication. Further analysis of nonessential and essential genes of cytomegalovirus will be needed to understand CMV viral pathology and to develop vaccines for herpesvirus infection and vectors for gene therapy.