Figure 12.

SIG5 expression control in the light-signaling pathway regulating the expression of the chloroplast genes psbD and psbA for the photoreaction center II core proteins D1 and D2 in response to light intensity and quality. Light signals regulate SIG5 expression by photoreceptors in a HY5- and COP1-controlled manner via various promoter elements. SIG5 is post-translationally targeted to chloroplasts, where the sigma factor is involved in tuning expression of psbD and psbA. Photoreaction center expression is involved in the PSII repair cycle. Retrograde signals transmit information correlating with the photosystem II excitation status and damage, respectively, to the nucleus to adjust the intensity of SIG5 transcription.