PEROXISOMES IN THE METHYLOTROPHIC YEAST HANSENULA-POLYMORPHA DO NOT NECESSARILY DERIVE FROM PREEXISTING ORGANELLES

We have identified two temperature-sensitive peroxisome-deficient muta nts of Hansenula polymorpha (ts6 and ts44) within a collection of ts m utants which are impaired for growth on methanol at 43-degrees-C but g row well at 35-degrees-C. In both strains peroxisomes were completely absent in cells grown at 43-degrees-C; the major peroxisomal matrix en zymes alcohol oxidase, dihydroxyacetone synthase and catalase were syn thesized normally but assembled into the active enzyme protein in the cytosol. As in wild-type cells, these enzymes were present in peroxiso mes under permissive' growth conditions (less-than-or-equal-to 37-degr ees-C). However, at intermediate temperatures (38-42-degrees-C) they w ere partly peroxisome-bound and partly resided in the cytosol. Genetic analysis revealed that both mutant phenotypes were due to monogenic r ecessive mutations mapped in the same gene, designated PER13. After a shift of per13-6ts cells from restrictive to permissive temperature, n ew peroxisomes were formed within 1 h. Initially one-or infrequently a few-small organelles developed which subsequently increased in size a nd multiplied by fission during prolonged permissive growth. Neither m ature peroxisomal matrix nor membrane proteins, which were present in the cytosol prior to the temperature shift, were incorporated into the newly formed organelles. Instead, these proteins remained unaffected (and active) in the cytosol concomitant with further peroxisome develo pment. Thus in H.polymorpha alternative mechanisms of peroxisome bioge nesis may be possible in addition to multiplication by fission upon in duction of the organelles by certain growth substrates.