Identification and initial characterization of 2nd site suppressors of pdr16 mutant medium chain fatty acid‐sensitive phenotype

Abstract

Previously, we have found that PDR16 is required for normal growth in the presence of medium chain fatty acids. We found that cells that have a mutant pdr16 gene contain increased content of fatty acids and sterols, and exhibit several characteristic changes in the corresponding lipid compositions. Most notably, pdr16 mutation results in a 50% reduction in the percentage of shorter chain fatty acids (C10-C14), a greater than 50% reduction in the percentage of the very long chain fatty acid C26:0, and an increase of about 6 percentage points in the amount of C18:1. To better understand and identify the role that Pdr16p plays in the metabolism and/or trafficking of exogenous fatty acids, we undertook a screen for 2nd site mutations that suppress the fatty acid-sensitive mutant phenotype of the pdr16 mutant. Six 2nd site suppressor mutants were initially identified, 3 dominant and 3 recessive. Comparison of the phenotypes, and the lipid data for all 6 suppressor mutants, collectively, revealed an apparent correlation between the quality of suppression and certain lipid measurements. We found that those suppressors exhibiting a qualitatively greater degree of suppression also contained reduced amounts of fatty acids and sterols and elevated percentage of C26:0 compared to the pdr16 null mutant. However, the relative percentage of 18:1 in those mutants still remained high compared to the wild-type strain, W303-1A. In contrast, the two “weakest” suppressor mutants both contained fatty acids and sterols at levels nearer that measured in the pdr16 null mutant, had no increase in percentage of C26:0, but did show reduced percentage of 18:1 to levels similar to that detected in wild type.