Restoration of peroxisome formation in two conditional peroxisome-deficient mutants of Hansenula polymorpha during growth of cells on specific organic nitrogen sources

Expression of the peroxisome-deficient (Per^?) phenotype by per mutants of Hansenula polymorpha is shown to be dependent on specific environmental conditions. Analysis of our collection of constitutive and conditional per mutants showed that, irrespective of the carbon source used, the mutants invariably lacked functional peroxisomes when ammonium sulphate was used as a nitrogen source. However, in two temperature-sensitive (ts) mutants, per13-6^ts and per14-11^ts, peroxisomes were present at the restrictive temperature when cells were grown on organic nitrogen sources which are known to induce peroxisomes in wild-type cells, namely D -alanine (for both mutants) or methylamine (for per14-11^ts). These organelles displayed normal wild-type properties with respect to morphology, mode of development and protein composition. However, under these conditions not all the peroxisomal matrix proteins synthesized were correctly located inside peroxisomes. Detailed biochemical and (immuno) cytochemical analyses indicated that during growth of cells on methanol in the presence of either D -alanine or methylamine, a minor portion of these proteins (predominantly alcohol oxidase, dihydroxyacetone synthase and catalase) still resided in the cytosol. This residual cytosolic activity may explain the observation that the functional restoration of the two ts mutants is not complete under these conditions, as is reflected by the retarded growth of the cells in batch cultures on methanol.