Isolation of actin-encoding cDNAs from symbiotic corals.

A cDNA (named LGfact) encoding actin was identified in planular larvae of the scleractinian coral Galaxea fascicularis, using the reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) techniques. RNA from the adult coral that was inhabited by symbiotic dinophytes was subjected to a similar RT-PCR, and a cDNA fragment, named AGfact-p, was found to encode an actin form distinct from LGfact. In an expression study, LGfact transcripts were present at similar levels in asymbiotic larvae and symbiotic adults, indicating that LGfact was expressed by the host. On the other hand, the expression of AGfact-p was detected in adults but not in larvae. Partial cDNA sequences of orthologues of LGfact and AGfact-p were detected in another scleractinian coral, Favites chinensis. A sequence identical to a part of AFcact-p (an AGfact-p orthologue) was amplified from the genomic DNA extracted from asymbiotic larvae of F. chinensis, strongly suggesting that AFcact-p was a coral actin cDNA. Thus, we presume that A Gfact-p encodes an adult-specific form of actin in the host. A partial actin-encoding cDNA sequence (named Syact-p) obtained from Symbiodinium sp. did not exhibit high levels of similarity to the coral actin sequences.