Sequential cold-sensitive mutations in Aspergillus fumigatus. III. Mechanism of cold sensitivity

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Abstract
The mechanism of cold sensitivity of Aspergillus fumigatus ON5, a 37 °C-sensitive mutant derived from A. fumigatus I-21 (ATCC 32722) by five sequential mutations, was investigated. The rate of in vivo protein synthesis by ON5 was not affected for 2 h following a shift from 45 to 34 °C, but the rate of in vivo RNA synthesis dropped almost immediately. The RNA polymerases of ON5 possessed wild-type activity in vitro at a nonpermissive temperature (34 °C) indicating that the reduction in the rate of in vivo RNA synthesis did not result from cold sensitivity in transcription, but was possibly a result of rapid feedback inhibition of transcription. Mutant ON5 was not able to produce ribosomes at a nonpermissive temperature as evidenced by the fact that no 3H-labelled amino acids were incorporated into the monosome, large ribosomal subunit, or small ribosomal subunit at 34 °C. Ribosomal subunit assembly or ribosomal RNA processing appears, therefore, to be the cold-sensitive cellular function in ON5.