SUMMARY: During anaerobic growth of the specific RNA content (μg. RNA/μg. protein) of cultures was directly proportional to the light intensity in which they were grown. Cultures subjected to a step-up in light intensity preferentially synthesized RNA until their specific RNA content increased to the value characteristic of growth at the higher light intensity. Conversely following a step-down in light intensity the rate of RNA synthesis fell below the rate of protein synthesis, and cellular RNA was diluted out to the value characteristic of growth at the lower light intensity. Adjustment of the differential rate of RNA synthesis in response to a change in light intensity was thus opposite to that of the differential rate of photopigment synthesis. Although a decrease in light intensity drastically decreased RNA synthesis some RNA continued to be formed. The RNA synthesized, like that in cultures maintained at a constant light intensity, consisted of soluble and ribosomal RNA and unstable RNA with sedimentation behaviour like m-RNA. Aerobic cultures of subjected to a marked decrease in O tension synthesized photopigments without appreciably increasing their net content of RNA, but continued to synthesize small amounts of all the usual classes of RNA.


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