The morphology of Mucor racemosus in cultures continuously sparged with nitrogen gas was investigated. When appropriate precautions were taken to prevent oxygen from entering the cultures, the morphology of the cells was uniformly yeastlike irrespective of the N2 flow rate. When small amounts of oxygen entered the cultures the resulting microaerobic conditions evoked mycelial development. Polypeptides synthesized by aerobic mycelia, microaerobic mycelia, anaerobic yeasts, and yeasts grown in a CO2 atmosphere were compared by two-dimensional gel electrophoresis. The results indicated that a large number of differences in polypeptide expression exist when microaerobic mycelia or anaerobic yeasts are compared with aerobic mycelia and that these alterations correlate with a change from an oxidative to a fermentative metabolic mode. Relatively few differences in polypeptide composition exist when microaerobic cells are compared with anaerobic cells, but these changes correlate with a change from the mycelial to the yeast morphology. We hypothesize that oxygen regulates the expression of polypeptides involved in both the metabolic mode and in morphogenesis.

A conditional developmental mutant of Mucor racemosus which is capable of oxidative energy metabolism is described. Unlike the wild-type strain the mutant was highly fermentative and exhibited the yeast morphology when grown aerobically in glucose-containing media. The high fermentative activity and yeast morphology under these conditions correlated well with maximal expression of glycolytic enzymes and with expression of some polypeptides' characteristic of anaerobic growth. Aerobic growth of the mutant on amino acids as the sole carbon source resulted in growth in the' mycelial morphology. The mnutant was fully capable'of oxidative metabolism as judged by its ability to grow on amino acids, respiratory capacity, and complement of tricarboxylic acid cycle enzymes. The results support the hypothesis that oxygen controls both the expression of glycolytic enzymes and the expression of proteins involved in morphogenesis. Moreover, they suggest that there are common regulatory elements in the control of these two classes of gene products. Abnormally high levels of aconitase and isocitrate dehydrogenase in the mutant are consistent with the proposal that pool sizes of citrate may act as a regulator of genes responsive to environmental oxygen concentration.

The year was 1957. The United States, a world power arising out of the ashes of World War II, was entering an age of economic prosperity. Especially in the United States, the many successes of science had created optimism about a new age of technological and social well-being. But at the same time, Sputnik had just been launched by the Soviet Union. The Cold War was in its second decade and still growing. The civil rights era was just beginning in the United States, where social and economic disparities were about to generate a war on poverty.Amidst this social context, Donald T. Campbell, having recently been told he was unlikely to be tenured at the University of Chicago because of his sparse publication record (Campbell, 1981), was in his third academic position, an associate professorship in psychology at Northwestern University. At Northwestern, Campbell continued to pursue several related programs of research that he hoped would eventually garner him both academic tenure and the respect of his colleagues. 1