Allelic Complementation in the First Gene for Histidine Biosynthesis in SACCHAROMYCES CEREVISIAE. II. Complementation Mapping of Mutants and a Subunit Model of the Enzyme.

Sixty-two alleles of the histidine-1 (his1) gene were tested for complementation. The 44 complementing mutants fell into 31 complementation groups which were used to construct a complex complementation map with 18 complementation units. Cluster analysis of the complementation map by either visual inspection or the computer method of Gillie and Peto (1969) shows two very definite clusters.The molecular weight estimate of the his1 enzyme, phosphoribosyl adenosine triphosphate: pyrophosphate phosphoribosyltransferase, is 1.8 . 10(5) by sucrose density gradient analysis and 2.4 . 10(5) by Sephadex gel chromatography. Correlating the length of the his1 gene to the molecular weight of the enzyme indicates that this enzyme is composed of 6 subunits, as is the analogous enzyme in Salmonella typhimurium.A model of the subunit and tertiary and quaternary structure of the enzyme has been developed from consideration of the genetic and complementation data, the distribution of the various mutant types within the gene, and the biochemical properties of the enzyme encoded by the his1 gene.