Background: Saccharomyces cerevisiae sterol gene expression is regulated by a consensus sterol-response promoter element (SRE/AR1 c ). Results: The anaerobic AR1 b promoter element regulates global antifungal-dependent sterol gene expression. Conclusion: Yeast sterol gene expression is regulated by multiple SRE-like elements. Significance: Understanding sterol gene expression will yield valuable information concerning antifungal drug resistance.

Certain minor minicircle sequence classes in the kinetoplast DNA (kDNA) networks of arsenite-or tunicamycin-resistant Leishmania mexicana amazonensis variants whose nuclear DNA is amplified appear to be preferentially selected to replicate (S. T. Lee, C. Tarn, and K. P. Chang, Mol. Biochem. Parasitol. 58:187-204, 1993). These sequences replace the predominant wild-type minicircle sequences to become dominant species in the kDNA network. The switch from wild-type-specific to variant-specific minicircles takes place rapidly within the same network, the period of minicircle dominance changes being defined as the transition period. To investigate the structural organization of the kDNA networks during this transition period, we analyzed kDNA from whole arsenite-resistant Leishmania parasites by dot hybridization with sequence-specific DNA probes and by electron-microscopic examination of isolated kDNA networks in vitro. Both analyses concluded that during the switch of dominance the predominant wild-ype minicircle class was rapidly lost and that selective replication of variant-specific minicircles subsequently filled the network step by step. There was a time during the transition when few wild-type-or variant-specific minicircles were present, leaving the network almost empty and exposing a species of thick, long, fibrous DNA which seemed to form a skeleton for the network. Both minicircles and maxicircles were found to attach to these long DNA fibrils. The nature of the long DNA fibrils is not clear, but they may be important in providing a framework for the network structure and a support for the replication of minicircles and maxicircles.

An A + T-rich repeated DNA sequence cloned from maxicircle DNA of Leishmania mexicana amazonensis was found to be conserved in the maxicircles of human-infective Leishmania species but totally absent from non-human-infective ones. Differences in the pattern of distribution of this repeated sequence in the maxicircles of each human-infective Leishmania species potentially allow the development of a sensitive probe to differentiate among these species.

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