A light-activated DNA-binding factor stimulates transcription of the rrnA operon in the cyanobacterium Synechococcus sp. PCC 6301.

In Synechococcus sp. strain PCC 6301, ribosomal RNA (rRNA) synthesis occurs at specific times during the growth cycle in the light. When light-grown cultures are placed in the dark, rRNA synthesis and cell division stop abruptly. It is shown here that a partially purified DNA-binding protein binds downstream of the rRNA operon (rrnA) P1 promoter in the light but not in the dark. When the DNA binding protein is added to in vitro transcription assays, run-off transcripts are produced in the light but not, under dark conditions. The results indicate that a light-activated regulatory molecule is involved in stimulating rRNA synthesis during the normal cell growth cycle of Synechococcus in the light.