Localization of transcription in nucleoli of rat sympathetic neurons. A quantitative ultrastructural autoradiography study.

A quantitative electron microscopy autoradiography method was used to locate the sites of initial rRNA transcription and to determine whether there was a variability of fibrillar centres size related to nucleolar transcription in nucleoli of sympathetic neurons of a rat killed during the dark span of its light-dark cycle. Indeed, in this model two types of fibrillar centres (nucleolar organizing regions) were observed: a single large type characteristic of this phase of the photoperiod and a small type composed by several small fibrillar centres of normal size. The present results clearly demonstrated that: there was no incorporation into the fibrillar centres themselves; the rRNA transcription sites were restricted to the ribonucleoprotein (RNP) fibrillar component and the products thereafter migrated into the RNP granular component. There was no variability of fibrillar centres size related to nucleolar transcription. The level of transcription was found to be identical around the two types of fibrillar centres. They might be considered as equivalent. The results provide evidence that at least in this model, the size of fibrillar centres and the transcriptional activity are not correlated. Other factors must therefore be responsible for the difference in size.