Position-effect variegation--an assay for nonhistone chromosomal proteins and chromatin assembly and modifying factors.

The past decade has brought us much information about gene regulation in eukaryotes and the importance of chromatin in gene expression. Clearly, we need to advance our understanding of chromatin assembly and modification. Hopefully, many Su(var) and E(var) loci will be cloned and their analysis will demonstrate that they are involved in this process. It will be interesting to see what classes of products they encode. Clearly, antibodies must be made against these proteins to determine: (1) the chromosomal distribution of those that encode chromatin structural proteins, (2) the tissue and temporal distribution of these proteins during development, and (3) the time of action during specific portions of the cell cycle for those genes encoding factors that regulate chromatin assembly or modifying proteins. In addition, it would be intriguing to identify and isolate those sequences or segments of DNA that serve as nucleation sites for the assembly of euchromatin and heterochromatin. The necessary materials for identifying these sequences are in place with the large number of variegating strains and revertants of those rearrangements and Su(var) and E(var) mutations that exist. From our current understanding of PEV, it appears that PEV provides a useful model for the study of determinative events that occur during development. This includes both the establishment of different lineages and the somatic memory or imprinting of the initial determinative decision. Clearly, PEV is more than a simple assay system for the identification of NHCPs, and ultimately the understanding of the mechanisms underlying PEV should lend insights into how such determinative events occur and are propagated.