Characterization of the gene encoding alpha-sarcin, a ribosome-inactivating protein secreted by Aspergillus giganteus

The filamentous fungus, Aspergillus giganteus, produces the extracellular ribosome-inactivating protein, alpha-sarcin (Sar). The structural gene (sar) encoding Sar was isolated and characterized by sequence analysis and expression in Aspergillus niger. It codes for a precursor of 177 amino acids containing a secretion signal sequence that is absent in the mature protein. The nucleotide sequence contains several typical features of fungal genes, including a short intron of 65 bp. The transcriptional and translational processing signals of the gene are functional in A. niger, but the yield of recombinant protein is low in comparison with the natural producer. A comparison of sar with the gene encoding restrictocin from Aspergillus restrictus revealed a high degree of similarity between both genes. The conservation of the aa sequence suggests that Sar-like proteins may confer a selective advantage to these fungi under certain environmental conditions.