STRUCTURE, EXPRESSION AND PHYLOGENETIC ANALYSIS OF THE GENE ENCODING ACTIN-I IN PNEUMOCYSTIS-CARINII

Actin is a major component of the cytoskeleton and one of the most abu ndant proteins found in eukaryotic cells. Comparative sequence analysi s shows that this essential gene has been highly conserved throughout eukaryotic evolution making it useful for phylogenetic analysis. Compl ete cDNA clones for the actin-encoding gene were isolated and characte rized from Pneumocystis carinii purified from immunosuppressed rat lun gs. The nucleotide sequence encodes a protein of 376 amino acids. The predicted actin protein of P. carinii shares a high degree of conserva tion to other known actins. Only one major actin gene was found in P. carinii. The P. carinii actin sequence was compared with 30 other acti n sequences. Gene phylogenies constructed using both neighbor-joining and protein parsimony methods places the P. carinii actin sequence clo sest to the majority of the fungi. Since the phylogenetic relationship of P. carinii to fungi and protists has been questioned, these data o n the actin gene phylogeny support the grouping of P. carinii with the fungi.