Photoactivated adenylyl cyclase controls phototaxis in the flagellate Euglena gracilis.
Euglena gracilis, a unicellular freshwater protist exhibits different photomovement responses, such as phototaxis (oriented movement toward or away from the light source) and photophobic (abrupt turn in response to a rapid increase [step-up] or decrease [step-down] in the light fluence rate) responses. Photoactivated adenylyl cyclase (PAC) has been isolated from whole-cell preparations and identified by RNA interference (RNAi) to be the photoreceptor for step-up photophobic responses but not for step-down photophobic responses (M. Iseki, S. Matsunaga, A. Murakami, K. Ohno, K. Shiga, C. Yoshida, M. Sugai, T. Takahashi, T. Hori, M. Watanabe [2002] Nature 415: 1047-1051). The present study shows that knockdown of PAC by RNAi also effectively suppresses both positive and negative phototaxis, indicating for the first time that PAC or a PAC homolog is also the photoreceptor for photoorientation of wild-type E. gracilis. Recovery from RNAi occurred earlier for step-up photophobic responses than for positive and negative phototaxis. In addition, we investigated several phototaxis mutant strains of E. gracilis with different cytological features regarding the stigma and paraxonemal body (PAB; believed to be the location for the phototaxis photoreceptor) as well as Astasia longa, a close relative of E. gracilis. All of the E. gracilis mutant strains had PAC mRNAs, whereas in A. longa, a different but similar mRNA was found and designated AlPAC. Consistently, all of these strains showed no phototaxis but performed step-up photophobic responses, which were suppressed by RNAi of the PAC mRNA. The fact that some of these strains possess a cytologically altered or no PAB demonstrates that at least in these strains, the PAC photoreceptor responsible for the step-up photophobic responses is not located in the PAB.
Ntefidou
Maria
M
Department of Plant Ecophysiology, Friedrich-Alexander University, Staudtstrasse 5, 91058 Erlangen, Germany.
Iseki
Mineo
M
Watanabe
Masakatsu
M
Lebert
Michael
M
Häder
Donat-Peter
DP
D000262:Adenylyl Cyclases / Q000378:metabolism* / Q000528:radiation effects; D000818:Animals; D001483:Base Sequence; D002736:Chloroplasts / Q000502:physiology; D017931:DNA Primers; D004789:Enzyme Activation; D005056:Euglena gracilis / Q000502:physiology* / Q000528:radiation effects; D005407:Flagella / Q000502:physiology*; D007700:Kinetics; D008027:Light; D009068:Movement / Q000502:physiology / Q000528:radiation effects; D010786:Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects; D013997:Time Factors
D017931:DNA Primers; D000262:Adenylyl Cyclases