The early genetic response to light in the green unicellular alga Chlamydomonas eugametos grown under light/dark cycles involves genes that represent direct responses to light and photosynthesis.
In the green unicellular alga Chlamydomonas eugametos, cellular division is readily synchronized by light/dark cycles. Under these conditions, light initiates photosynthetic growth in daughter cells and begins the G1 phase. Genes whose expression is regulated upon illumination are likely to be important mechanisms controlling cell proliferation. To identify some of those genes, two cDNA libraries were prepared with poly(A)+ extracted from cells either stimulated with light for 1 h or held in darkness (quiescent cells) during the same period. To restrict our analysis to those genes that are part of the primary response, cells were incubated in presence of cycloheximide. Differential screening of approximately 40,000 clones in each library revealed 44 clones which hybridize preferentially with a [32P] cDNA probe derived from RNA of light-stimulated cells and 15 clones which react selectively with a [32P] cDNA probe synthesized from poly(A)+ RNA of quiescent cells. Cross-hybridization of these clones identified 4 independent sequences in the light-induced (LI) collection and 2 in the uninduced (LR) library. Four of these cDNAs correspond to mRNAs that are positively or negatively regulated upon activation of photosynthesis. One clone represents a mRNA that accumulates transitorily at both transitions. Finally, LI818 cDNA identifies a new chlorophyll a/b-binding (cab) gene family whose mRNA accumulation is controlled by light and a circadian oscillator. The endogenous timing system controls LI818 mRNA accumulation so that it precedes the onset of illumination by a few hours. On the other hand, light affects LI818 mRNA levels independently of active photosynthesis.
Département de Biochimie, Faculté des Sciences et de Génie, Université Laval, Québec, Canada.
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Guertin
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002453:Cell Cycle; D002696:Chlamydomonas / Q000235:genetics* / Q000378:metabolism / Q000528:radiation effects; D004247:DNA / Q000235:genetics; D015342:DNA Probes; D003624:Darkness; D005786:Gene Expression Regulation / Q000528:radiation effects; D015723:Gene Library; D008027:Light; D045342:Light-Harvesting Protein Complexes; D008969:Molecular Sequence Data; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics*; D011061:Poly A / Q000235:genetics / Q000302:isolation & purification; D014176:Protein Biosynthesis; D012313:RNA / Q000235:genetics / Q000302:isolation & purification; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012689:Sequence Homology, Nucleic Acid; D014158:Transcription, Genetic
D015342:DNA Probes; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D012333:RNA, Messenger; D011061:Poly A; D012313:RNA; D004247:DNA