Protein synthesis during morphogenesis of Mucor racemosus

Abstract

Cells of M. racemosus were labeled with L-[14C]leucine during the yeast-to-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. 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 of free 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-[14C]leucine to a culture. Experiments performed 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 L-arginine produced results qualitatively the same as with L-leucine. The accumulation of incorporated L-[14C]leucine in a culture of M. racemosus undergoing the air-induced yeast-to-hyphae transition reflected the change in growth rate that accompanied the morphogenesis. However, the specific rate of protein synthesis measured throughout the developmental process displayed a characteristic acceleration during the emergence of germ tubes which was followed by a decline when all further growth took the form of hyphal elongation. Data are presented suggesting that this response is a correlate of morphogenesis rather than a consequence of the atmospheric change per se.