Phylogenetic analysis of NAP, an unconventional actin of the Volvocales
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The unicellular green alga Chlamydomonas reinhardtii has two actin genes, one encoding a conventional actin and the other an unconventional actin called novel actin-like protein (NAP). These actins apparently differ in their ability to polymerize, but their specific functions in the cell are unknown. To understand their evolutionary relationship, we investigated the molecular phylogeny of the actin/NAP family in Volvocales, using fully identified sequences (Chlamydomonas moewusii, Gonium pectorale, and Volvox carteri) and newly determined partial sequences (Eudorina elegans, and Volvulina steinii). Although the origin of the NAP clade remains ambiguous, the inferred phylogenetic trees strongly support the monophyly of NAP genes and show that only the genomes of volvocine species contain NAP genes. The nonsynonymous substitution rate of the NAP gene is, in consistence with its long branch length in the phylogeny, relatively high compared with that of the actin gene. NAP is thus apparently unique to volvocine species and most likely performs a cellular function with fewer constraints.
KeywordsVolvocales Divergent actin
Novel actin-like protein
Operational taxonomic unit
This work was supported by grants from Chuo University (Joint Research Grant and Grant for Special Research). The authors express gratitude to Drs. T. Hamaji, H. Nozaki, and A. Fujiyama for the sequence information of G. pectorale actin and NAP; Mses. Y. Kakiyama and Y. Ohto for their assistance with the experiments; Mr. T. Naito for sequencing of G. pectorale NAP cDNA; Dr. N Ueki for maintaining volvocine strains; and Dr. R. Kamiya for his critical reading of the manuscript.
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