Growth-rate-dependent adjustment of ribosome function in the fungus Mucor racemosus.

The dimorphic fungus Mucor racemosus was grown at rates between 0.043 and 0.434 doubling/h while maintained as yeasts or at rates between 0.21 and 0.50 doubling/h while maintained as hyphae by altering the composition of the growth medium or the gaseous environment of the cells. Yeasts at the higher growth rates contained many more ribosomes than did yeasts at the lower growth rates. They also had a higher percentage of ribosomes active in protein synthesis and a faster rate of polypeptide-chain elongation than did the slower-growing cells. Hyphal cells at faster growth rates also contained many more ribosomes and showed a faster rate of polypeptide-chain elongation than did slower-growing cells. However, the faster-growing cells had a substantially lower proportion of ribosomes active in protein synthesis than did the slower-growing hyphae. Pulse-chase experiments failed to provide any evidence of protein turnover, which might otherwise invalidate the values calculated for the peptide-chain elongation rates.