Description:
<jats:title>Abstract</jats:title><jats:p>The <jats:styled-content style="fixed-case">AIDS</jats:styled-content>‐associated lung pathogen <jats:italic>Pneumocystis</jats:italic> is classified as a fungus although <jats:italic>Pneumocystis</jats:italic> has several distinct features such as the absence of ergosterol, the major sterol of most fungi. The <jats:italic>Pneumocystis carinii S</jats:italic>‐adenosylmethionine:sterol C24‐methyltransferase (<jats:styled-content style="fixed-case">SAM</jats:styled-content>:<jats:styled-content style="fixed-case">SMT</jats:styled-content>) enzyme, coded by the <jats:italic>erg6</jats:italic> gene, transfers either one or two methyl groups to the C‐24 position of the sterol side chain producing both C<jats:sub>28</jats:sub> and C<jats:sub>29</jats:sub> 24‐alkylsterols in approximately the same proportions, whereas most fungal <jats:styled-content style="fixed-case">SAM</jats:styled-content>:<jats:styled-content style="fixed-case">SMT</jats:styled-content> transfer only one methyl group to the side chain. The sterol compositions of wild‐type <jats:italic>Sacchromyces cerevisiae</jats:italic>, the <jats:italic>erg6</jats:italic> knockout mutant (<jats:italic>Δerg6</jats:italic>), and <jats:italic>Δerg6</jats:italic> expressing the <jats:italic>P. carinii</jats:italic> or the <jats:italic>S. cerevisiae erg6</jats:italic> gene were analyzed by a variety of chromatographic and spectroscopic procedures to examine functional complementation in the yeast expression system. Detailed sterol analyses were obtained using high performance liquid chromatography and proton nuclear magnetic resonance spectroscopy (<jats:sup>1</jats:sup>H‐<jats:styled-content style="fixed-case">NMR</jats:styled-content>). The <jats:italic>P. carinii </jats:italic><jats:styled-content style="fixed-case">SAM</jats:styled-content>:<jats:styled-content style="fixed-case">SMT</jats:styled-content> in the <jats:italic>Δerg6</jats:italic> restored its ability to produce the C<jats:sub>28</jats:sub> sterol ergosterol as the major sterol, and also resulted in low levels of C<jats:sub>29</jats:sub> sterols. This indicates that while the <jats:italic>P. carinii </jats:italic><jats:styled-content style="fixed-case">SAM</jats:styled-content>:<jats:styled-content style="fixed-case">SMT</jats:styled-content> in the yeast <jats:italic>Δerg6</jats:italic> cells was able to transfer a second methyl group to the side chain, the action of Δ<jats:sup>24(28)</jats:sup>‐sterol reductase (coded by the <jats:italic>erg4</jats:italic> gene) in the yeast cells prevented the formation and accumulation of as many C<jats:sub>29</jats:sub> sterols as that found in <jats:italic>P. carinii</jats:italic>.</jats:p>