The pathogenic yeast, Candida albicans, is insensitive to the anti-mitotic drug, benomyl, and to the dihydrofolate reductase inhibitor, methotrexate. Genes responsible for the intrinsic drug resistance were sought by transforming Saccharomyces cerevisiae, a yeast sensitive to both drugs, with genomic C. albicans libraries and screening on benomyl or methotrexate. Restriction analysis of plasmids isolated from benomyl- and methotrexate-resistant colonies indicated that both phenotypes were encoded by the same DNA fragment. Sequence analysis showed that the fragments were nearly identical and contained a long open reading frame of 1694 bp (ORF1) and a small ORF of 446 bp (ORF2) within ORF1 on the opposite strand. By site-directed mutagenesis, it was shown that ORF1 encoded both phenotypes. The protein had no sequence similarity to any known proteins, including beta-tubulin, dihydrofolate reductase, and the P-glycoprotein of the multi-drug resistance family. The resistance gene was detected in several C. albicans strains and in C. stellatoidea by DNA hybridization and by the polymerase chain reaction.

Three tripeptidyl polyoxins were synthesized and found to inhibit Candida albicans. Compared with the naturally occurring polyoxin D, the three synthetic polyoxins had little effect on chitin synthetase when assayed with a C. albicans membrane preparation. However, all the compounds inhibited growth, affected cell morphology in a manner similar to that of polyoxin D, and were hydrolyzed by cell extracts of C. albicans. Hydrolysis did not occur extracellularly, and at least one of the synthetic polyoxins, leucyl-norleucyl-uracil polyoxin C, inhibited peptide uptake, suggesting entrance into the cell via the peptide transport system. Thus, the intact tripeptidyl polyoxins are inactive prodrugs that are converted to active moieties by cellular enzymes.

Recombinant DNA molecules containing CDNA to a soybean mosaic virus (SMV) RNA genome were constructed and partial nucleotide sequences determined for two cDNA inserts, pSMV-34 and pSMV-35. Comparison of the predicted amino acid sequence encoded by the pSMV-34 cDNA insert to other potyvirus protein sequences revealed extensive homology with the region of the genome encoding the NIa proteinase, with conservation of the amino acids proposed to form the catalytic triad of the active site. Cell-free transcription and translation of the cloned cDNA sequence containing the NIa open reading frame and flanking sequences revealed that NIa proteinase sequences, which were expressed as part of a high Mr precursor, were able to undergo proteolytic processing. Alteration of the codon for one of the putative active site residues by site-directed mutagenesis eliminated processing and resulted in the accumulation of a high Mr precursor. Based on predicted amino acid sequences at five putative cleavage sites within the SMV polyprotein, a consensus SMV NIa proteinase cleavage sequence of Glu/AsnXaa-Val-Xaa-Xaa-Gln~Gly/Ser was proposed. The SMV NIa proteinase and its putative cleavage sites maintained motifs found in other potyviruses.

The synthesis and biological properties of seven polyoxins (4-10) designed to avoid peptidase hydrolysis in Candida albicans are presented. Five dipeptidyl and two tripeptidyl polyoxin analogues were synthesized by coupling an amino acid active ester or azlactone to uracil polyoxin C (2) or polyoxin D (1), subsequent removal of the protecting group, and purification by preparative HPLC. A new and novel route for introducing an n-propyl group onto the alpha-amino group of peptides is reported. With the exception of a carboxamide derivative, 8, all analogues were resistant to hydrolysis by a cell extract or permeabilized cells of Candida. Chitin synthetase inhibition constants were determined for 4-10 and the KI values ranged from 7.15 X 10(-6) M for octanoyl-phenylalanyl-polyoxin D (10) to 1.06 X 10(-3) M for D-tryptophanyl-uracil polyoxin C (6). These novel polyoxins do not compete with the transport of either peptides or uridine into the cell. Millimolar concentrations of compounds 4-10 are required to inhibit growth, cause morphological alterations, or reduce the viability of C. albicans.

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