Several groups have reported the cloning and sequencing of genes invol
ved in the biogenesis of yeast peroxisomes. Yeast strains bearing muta
tions in these genes are unable to grow on carbon sources whose metabo
lism requires peroxisomes, and these strains lack morphologically norm
al peroxisomes. We report the cloning of Pichia pastoris PAS1, the hom
ologue (based on a high level of protein sequence similarity) of the S
accharomyces cerevisiae PAS1. We also describe the creation and charac
terization of P. pastoris pas1 strains. Electron microscopy on the P.
pastoris pas1 cells revealed that they lack morphologically normal per
oxisomes, and instead contain membrane-bound structures that appear to
be small, mutant peroxisomes, or ''peroxisome ghosts.'' These ''ghost
s'' proliferated in response to induction on peroxisome requiring carb
on sources (oleic acid and methanol), and they were distributed to dau
ghter cells. Biochemical analysis of cell lysates revealed that peroxi
somal proteins are induced normally in pas1 cells. Peroxisome ghosts f
rom pas1 cells were purified on sucrose gradients, and biochemical ana
lysis showed that these ghosts, while lacking several peroxisomal prot
eins, did import varying amounts of several other peroxisomal proteins
. The existence of detectable peroxisome ghosts in P. pastoris pas1 ce
lls, and their ability to import some proteins, stands in contrast wit
h the results reported by Erdmann et al. (1991) for the S. cerevisiae
pas1 mutant, in which they were unable to detect peroxisome-like struc
tures. We discuss the role of PAS1 in peroxisome biogenesis in light o
f the new information regarding peroxisome ghosts in pas1 cells.