California, Berkeley, that the bovine papilloma virus initiator proteins, E1 and E2, recognize their origin of replication cooperatively.

Second, Stillman suggests there may be important parallels between the mechanisms for initiating duplication in DNA and the mechanisms for controlling expression of the replicated DNA. Both may use an array of multipartite switches, in which a critical number are turned on before the system is set in operation. In particular, Stillman "proposes that an origin of DNA replication in yeast, element 'A,' provides an initiator function which, like the TATA box of a eukaryotic promoter, is essentially inactive unless stimulated by the other 'B' elements, called activator elements."

Discovery of the origin recognition complex will also allow other experiments that may reveal why the initiation of DNA replication at each origin is limited to one round only per cell cycle. It has been suggested that the initiator protein function is destroyed following each initiation event or that a newly replicated DNA strand is made insensitive to the presence of the initiator. With an initiator protein complex in hand, these ideas can be tested. In fact, Joachim Li of the University of California, San Francisco, and Bruce Alberts, formerly at that institution and now president of the National Academy of Sciences, have noted that, "By following the fate of the Saccharomyces cerevisiae initiator protein during the replication reaction and throughout the cell cycle, we can expect to learn how the cell prepares itself for a new round of initiation and how it prevents those preparations from occurring prematurely."

Another possibility is to use information about the yeast origin recognition protein to fill gaps in our understanding of the cell cycle (see Figure 4.6). For example, much attention has been given to the study of the various biochemical events of the G1 phase immediately prior to the synthesis of DNA. It is known that certain genes that are active during this phase produce enzymes known as kinases, which are important in the energy transfer reactions known as phosphorylations. Using the initiation protein complex as a starting point, it may be possible to work backwards to define a relationship between this complex and the important kinases produced during the preceding G1 phase. Perhaps some of the products produced in the G1 phase help with the synthesis or activity of one of the proteins that make up the origin recognition complex.

Still another strategy for learning more about the many proteins needed for eukaryotic DNA duplication is to work backwards from the yeast origin recognition complex to the genes that code for it. Joel A. Huberman, of the Roswell Park Cancer Institute, Buffalo, New York, has



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