ABSTRACT
More recent evidence supports classification of P. carin;; as a fungus. Most compelling is the molecular biological sequence analysis of P. carinii genes, including the 18s ribosomal RNA [23,24] and mitochondrial DNA [25] sequences which place the organism taxonomically among the fungi rather than the protozoa. The ultrastructure of the cyst wall of P. carinii resembles that of fungi [26], and P. carin;; and fungal cell walls share a common epitope identified by a monoclonal antibody [27]. P. carin;; also lacks some of the characteristic protozoan organelles, such as rhoptries, subpellicular tubules, and conoids [21]. P. carinii shows DNA sequence homology with the red yeast fungi [28]. The gene for elongation factor 3 (EF3), which is found exclusively in fungi, has been found in P. carinii [29]. In addition, thymidylate synthase (TS) and dihydrofolate reductase (DHFR) are two distinct enzymes in P. carinii [30,31], whereas in protozoa those activities are contained within a single bifunctional protein [32].
