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Ultrathin sections of Saccharomyces cerevisiae, grown on Lindegren's presporulation medium, have been studied with the electron microscope. A cytoplasmic membrane, mitochondria, nucleus, central vacuole, storage granules, and an internal double membrane system have been described. The cytoplasmic structures observed resemble closely analogous structures described in thin sections of cells from a variety of other sources.

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Studies of the photoheterotrophic, or nonsulfur purple, bacteria have shown that these microorganisms are closely related morphologically, and can be placed either in the genus Rhodospirillum or Rhodopseudomonas (van Niel, 1944). These two genera comprise the family Athiorhodaceae. In view of the morphological

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The gal3 mutation of Saccharomyces, which is associated with an impairment in the utilization of galactose, has been shown to be pleiotropic, causing similar impairments in the utilization of melibiose and maltose. Melibiose utilization and a-galactosidase production are directly controlled by the galactose regulatory elements i, c, and GAL4. The fermentation of maltose and the induction of a-glucosidase are regulated independently of the i,c,GAL4 system. The production of a-galactosidase and galactose-1-phosphate uridyl transferase is coordinate in galactokinaseless strains. Galactose serves as a nonmetabolized, gratuitous inducer of a-galactosidase in strains lacking the genes for one or more of the Leloir pathway enzymes.

HE genetic control of synthesis of the galactose pathway enzymes in Saccharomyces cerevisiae conforms in certain respects to the operon model proposed by JACOB and MONOD (1961) for the ,&galactosidase system of E. coli, and shown by BUTTIN (1963a, b) to be valid for the E. coli galactose system as well. Three closely linked structural genes specify the galactose pathway enzymes, galactokinase, galactose-1-phosphate-uridyl transferase (transferase), and uridine diphosphogalactose-4-epimerase (epimerase) (DOUGLAS and HAWTHORNE 1964). The three loci are under the control of an unlinked regulator gene, i, which is recognizable by its recessive mutations that permit constitutive synthesis of the three galactose enzymes (DOUGLAS and PELROY 1963).

Evolutionary divergence among species of the yeast genus Saccharomyces was estimated from measurements of deoxyribonucleic acid (DNA) /DNA and ribosomal ribonucleic acid (RNA) /DNA homology. Much diversity was found in the DNA base sequences with several species showing little or no homology to the three reference species, S. cerevisiae, S. lactis, and S. fragilis. These three reference species also showed little or no homology to each other. On the other hand the diversity among ribosomal RNA base sequences was small since most species showed a high degree of homology to the reference species. The arrangement of species based on ribosomal RNA homologies agrees in most cases with current taxonomic groupings. A yeast hybrid (S. fragilis X S. lactis) was shown to contain two nonhomologous genomes. A minimum genome size of 9.2 X 109 daltons for S. cerevisiae was calculated from the rate of DNA renaturation.