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The present study was undertaken in order to elucidate the molecular mechanisms responsible for regulating changes in the specific rate of protein synthesis during the yeast-to-hyphae morphogenesis in the fungus, Mucor racemosus. The distribution of ribosomes between active polysomes and monosomes and inactive subunits was determined by means of pulse-labeling and density gradient fractionation techniques. The percentage of ribosomes active in protein synthesis was observed to decrease throughout the morphological transition. The rate of amino acid addition to nascent polypeptide chains was calculated and the transit time of messenger RNA translation was measured. The results showed a significant increase in the velocity of ribosome movement along the message which was continuously adjusted throughout hyphal development.

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Cells of Mucor racemosus were labeled with L-[14C]leucine during the yeastto-hyphae morphogenesis that follows a change of atmosphere from CO2 to air. Pulse-labeling kinetics and the steady-state accumulation of incorporated L-[14C]leucine were determined throughout the period of cellular differentiation. We determined that the L-[14C]leucine was taken up by all forms of the organism, was not altered from the form of L-leucine, and was incorporated exclusively into protein. The intracellular pool offree L-leucine was small in comparison with those of the other L-amino acids, remained relatively constant in size during morphogenesis, and was rapidly equilibrated with exogenous leucine. Approximately the same internal radiospecific activities were attained throughout development shortly after addition of L-['4C]leucine to a culture. Experiments perforned with leucine auxotrophs suggested that endogenous synthesis of leucine in prototrophs does not affect the measured rates of incorporation. Experiments performed with 14C-labeled L-isoleucine, L-proline, L-lysine, and Larginine produced results qualitatively the same as with L-leucine. The accumulation of incorporated L-[14C]leucine in a culture ofM. racemosus undergoing the

Activities of the glyoxylate cycle enzymes isocitrate lyase (EC 4.1.3.1) and malate synthase (EC 4.1.3.2) were assayed in extracts prepared at different stages of myxospore formation in liquid cultures of Myxococcus xanthus. Activities of both enzymes attained peak values during conversion of rods to spheres. Isocitrate lyase activity decreased after reaching its peak value. Malate synthase activity also declined but at a much slower rate. The loss of isocitrate lyase activity could be prevented by the addition of chloramphenicol to cultures early in myxospore formation (during the initial rise in enzyme activity), but not by such addition at later stages of myxospore formation. The increase in glyoxylate cycle enzymes was not observed in a mutant unable to form myxospores in liquid culture under conditions suitable for morphological conversion of the wild type, or in wild-type cells incubated in the absence of an inducer for myxospore formation. It is concluded that the changes in the glyoxylate cycle enzymes represent regulatory phenomena associated with the development of the myxospore. MATERIALS AND METHODSOrganism. M. xanthus, strain FB (9), and a mutant (designated GNI) derived from FB were used. 784 on August 2, 2020 by guest http://jb.asm.org/ Downloaded from 788 ORLOWSKI ET AL.