Heat shock and light activation of aChlamydomonas HSP70 gene are mediated by independent regulatory pathways

Abstract

Induction of HSP70 heat shock genes by light has been demonstrated in Chlamydomonas. Our aim was to establish whether this induction by light is mediated by the heat stress sensing pathway or by an independent signal chain. Inhibitors of cytoplasmic protein synthesis revealed an initial difference. Cycloheximide and other inhibitors of protein synthesis prevented HSP70A induction upon illumination but not during heat stress. Analysis of HSP70A induction in cells that had differentiated into gametes revealed a second difference. While heat shock resulted in elevated HSP70A mRNA levels, light was no longer able to serve as an inducer in gametes. To identify the regulatory sequences that mediate the response of the HSP70A gene to either heat stress or light we introduced a series of progressive 5' truncations into its promoter sequence. Analyses of the levels of mRNA transcribed from these deletion constructs showed that in most of them the responses to heat shock and light were similar, suggesting that light induction is mediated by a light-activated heat shock factor. However, we show that the HSP70A promoter also contains cis-acting sequences involved in light induction that do not participate in induction by heat stress. Together, these results provide evidence for a regulation of HSP70A gene expression by light through a heat shock-independent signal pathway.

In Vivo Visualization of Mg-ProtoporphyrinIX, a Coordinator of Photosynthetic Gene Expression in the Nucleus and the Chloroplast

“…Furthermore, the relative cytoplasmic accumulation of Mg-ProtoIX and ProtoIX was consistently greater in the cotyledons compared with the hypocotyls (Figures 5N and 5O; see Supplemental Figures 2 and 3 online), suggesting that the export mechanism(s) is more active in leaf tissue compared with hypocotyls and that the export of tetrapyrroles is an active and regulated process. Supporting this conclusion, Kropat et al (1995Kropat et al ( , 1997Kropat et al ( , 2000 …”

“…Although there is clear biochemical (Kropat et al, 1995(Kropat et al, , 1997(Kropat et al, , 2000 and genetic (Susek et al, 1993;Larkin et al, 2003;Strand et al, 2003) evidence for Mg-ProtoIX-mediated plastid-tonucleus communication, it remains unclear how the signal is transmitted from the chloroplast, where it is synthesized, to the nucleus. Is Mg-ProtoIX emitted from the chloroplast itself or does it interact with chloroplast-localized signaling proteins that as a consequence are emitted from the chloroplast and transduce the plastid signal further?…”

Two enhancers and one silencer located in the introns of regA control somatic cell differentiation in Volvox carteri

“…In this context, we were intrigued by our earlier observation that the 1194-bp intron 7 is extremely unusual, in that it constitutes an ORF that is fully in frame with the ORFs of the two flanking exons , raising the question of whether intron 7 is ever retained in a mature transcript. This possibility was reinforced by the knowledge that intron 7 of regA (like three other volvocalean introns; Harper and Mages 1988;Fabry et al, 1993;Kropat et al 1995) has a GC, instead of the canonical GT, at the donor splice site, which could lead to inefficient splicing. However, our earlier Northern-blot analysis had revealed the presence of only one kind of processed transcript, a 7-kb component lacking intron 7 .…”

Chlorophyll precursors are signals of chloroplast origin involved in light induction of nuclear heat-shock genes

“…We have shown previously (21) that, for the light induction of these genes, a pathway is utilized that is different from the one used in response to heat shock. A clue to our biological understanding of the light induction of HSP70 genes was provided by the observation that, in cells incubated in the dark and then shifted to light intensities that caused photoinhibition, photosystem II was less damaged and recovered faster when, prior to light stress, the cultures were pre-exposed to dim light for 60 min.…”