DESCRIPTION (from the application): Replicative senescence or the terminal loss of proliferation exhibited by normal cells in culture is a well established phenomenon. We have demonstrated that the phenotype of cell senescence is dominant in fusions of normal with immortal cells. By fusing different immortal human cell lines with each other, we have identified four complementation groups (A-D) for indefinite division. Microcell mediated chromosome transfer has revealed the cell senescence related genes for groups B, C and D lie on human chromosomes 4, 1 and 7, respectively. We cloned the chromosome 4 gene, Mortality Factor on chromosome 4 (MORF4), and determined that it is a member of a novel family of transcription factor like genes. MORF4 is a truncated version of the MORF Related Gene on chromosome 15, MRGI 5, which has an additional 5' region that encodes a chromo domain motif. Chromo domain proteins are thought to affect transcription over large regions of chromosomes by remodeling chromatin structure. In the common regions of the two genes, there are many interesting motifs including a bipartite nuclear localization signal, a helix loop helix region, an area of similarity to the male specific lethal gene of Drosophila (MSL3) and a leucine zipper motif. These motifs are indicative of a role for these genes in transcriptional regulation. Our working hypothesis, based on preliminary data, is that MRG15 plays a role in cell cycle progression and cell senescence by activating expression of a number of genes by either a direct effect on transcription factors and/or by affecting chromatin structure. We will determine the components of protein complexes with which MRG15 is associated, determine which are important for activation of genes such as b-myb, determine whether MRG15 has a causal role in replicative senescence and determine the spectrum of genes transcription of which is affected by MRG15 complexes. The results should aid in our understanding of the mechanisms of action of both genes and contribute to our knowledge of senescence and immortalization.