[In vitro expression of the thyroid gland genome. Difference between chromatin and DNA]

In vitro, transcription by thyroid chromatin has been compared to that carried out by deproteinized DNA prepared from the same tissue. The chromatin-primed transcripts are represented by 4S and 10S RNAs, whereas the DNA-primed transcript is heterogenous and polydisperse. The sequences expressed in vitro have been analyzed by hydridization of 4S and 10S RNA to the total native thyroid DNA. Most of the sequences transcribed are represented by intermediate repetitive-associated an unique sequence. Furthermore, they are expressed only by chromatin and not by DNA. Thus, thyroid chromatin very likely holds in vitro the same fonctional restriction exhibited in vivo, whereas the transcriptional behaviour of thyroid DNA is totally aspecific.