Activates transcription. Positively regulates PcrtB promoter upstream of the crtB operon in a cAMP-independent manner. Regulated genes include genes encoding DNA photolyase, phytoene synthase and cytochrome P450 monooxygenase, which are involved in carotenoid biosynthesis. Positively regulates the light-inducible gene cluster in the megaplasmid in a cAMP-independent manner. Homodimer.