Chloroplast signalling in the light induction of nuclear HSP70 genes requires the accumulation of chlorophyll precursors and their accessibility to cytoplasm/nucleus.
Chlorophyll precursors Mg-protoporphyrin IX and its monomethylester are candidates for plastid-derived molecules involved in light signalling from the chloroplast to the nucleus. The pool sizes of these two Mg2+-containing porphyrins and of protoporphyrin IX transiently increased upon a shift of Chlamydomonas cultures from dark to light. This increase coincided with the accumulation of mRNAs encoded by the nuclear genes HSP70A and HSP70B. Analysis of a mutant (brs-1), previously shown to be defective in the light induction of these genes, revealed high levels of protoporphyrin IX but no light-induced increase in the levels of Mg2+-containing porphyrins. Inhibitors of cytoplasmic protein synthesis prevented both the light-induced rise in pool levels and induction of the HSP70 genes. Similarly, pre-gametes, intermediates of sexual differentiation, lacked both responses to light. The block in light induction of the HSP70 genes in inhibitor-treated cells and in pre-gametes could be circumvented by the exogenous addition of Mg-protoporphyrin IX in the dark. This suggests an essential role for light-induced Mg-protoporphyrin IX accumulation in this chloroplast-to-nucleus signalling pathway. However, accumulation of this porphyrin in the dark - presumably in the chloroplast - did not result in induction. A second crucial role for light in this signalling pathway is postulated which makes this plastidic compound accessible to the cytoplasm/nucleus where the downstream signalling pathway may be activated.
Kropat
J
J
Institut für Biologie III, Universität Freiburg, Schaenzlestrasse 1, D-79104 Freiburg, Germany.
Oster
U
U
Rüdiger
W
W
Beck
C F
CF
D000818:Animals; D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism*; D002696:Chlamydomonas / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D002736:Chloroplasts / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects / Q000528:radiation effects; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics*; D008027:Light; D009687:Nuclear Proteins / Q000235:genetics; D011166:Porphyrins / Q000378:metabolism; D011521:Protochlorophyllide / Q000378:metabolism; D011524:Protoporphyrins / Q000378:metabolism* / Q000494:pharmacology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000528:radiation effects; D013997:Time Factors
D018840:HSP70 Heat-Shock Proteins; D009687:Nuclear Proteins; D011166:Porphyrins; D011524:Protoporphyrins; D012333:RNA, Messenger; C039629:magnesium protoporphyrin; C033434:protoporphyrin IX monomethyl ester; D011521:Protochlorophyllide; C028025:protoporphyrin IX