Purified phytochrome influences in vitro transcription in rye nuclei

Nuclei isolated from dark-grown seedlings of rye by Percoll density gradient centrifugation incorporate radioactive UTP into RNA. Transcription is reduced to 50% by the addition of alpha-amanitin (5 mug/ml) and correspondingly a decrease of the label in the mRNA fraction is seen on agarose gels. Purified 124-kd phytochrome in its far-red absorbing form (P(fr)) increases the incorporation of labeled UTP by 40-70% above untreated controls, whereas pre-illumination with far-red light or addition of alpha-amanitin prevents this effect. Nuclei isolated from light-grown seedlings show only an increase of 5% upon addition of P(fr). Other proteins such as bovine serum albumin or cytochrome c do not enhance the rate of transcription. We conclude that the accumulation of mRNA species is influenced by phytochrome in its P(fr) form.