There is much to be learned about how chromosomes are faithfully transmitted during mitosis and meiosis. Determining the detailed structure of an entire eucaryotic chromosome should yield much information about its functions. As a first step towards this goal, we have been examining the structure of the 250 kb DNA molecule from chromosome I, the smallest Saccharomyces cerevisiae chromosome. We have cloned almost all of it and have determined the physical location of all but two of its classically identified genes on a restriction map of the entire chromosomal DNA molecule. In collaborative efforts, we have determined the transcription map on 160 kb of chromosome I and are in the process of locating all the cloned ARS elements. We propose to continue elucidating the transcription map of the entire chromosome. These studies will provide the foundation for further analyses on many aspects of chromosome function and behavior. Classical genetic studies have succeeded in identifying less than 20% of all yeast genes. In this respect, chromosome I is typical. Hybridization studies suggested that chromosome I encodes 90-100 genes. However, despite intense efforts, only 15 of these genes were identified by classically obtained mutations. These proportions also apply to essential genes. The number of essential genes identified by classical genetic studies throughout the entire yeast genome and on chromosome I in particular was far lower than predicted. If we are to understand the molecular biology of a simple organism, it would be advantageous to have a more complete set of mutations in genes essential for its life cycle. It is clear that molecular approaches are needed to obtain this large number of mutants. We have already produced deletion mutations in 28 genes that were defined only by the presence of an unidentified transcribed region. Five of these genes were essential for vegetative growth on rich medium. Counting the previously identified genes, we now have mutants for 41 of the 90-100 genes on this chromosome. Based on these results, we suggest that 18-24% (1100-1500) of the genes in yeast are essential for growth on rich medium. We propose to continue these studies in order to eventually obtain a catalogue of mutants for every gene on chromosome I. Deletion mutations in approximately 5 additional transcribed regions will be produced using one-step gene replacement and their phenotypes will be partially characterized. These studies will enhance our knowledge about a large number of genes that have not been investigated previously.
