The transcriptional activity of individual ribosomal DNA gene clusters is modulated by serum concentration.

The activity of ribosomal gene clusters has been studied by cytological methods in human cultured cells grown in different amounts of serum under controlled experimental conditions. It has been shown that increasing amounts of serum induce an increase in ribosomal RNA synthesis at the single cell level. Furthermore, the concomitant identification of individual rRNA gene clusters by fluorescent techniques allowed us to demonstrate: (1) that individual gene clusters have differential transcriptional activity and differential frequency of activation; (2) that ribosomal gene activity is closely associated with the amount of silver-positive gene product and; (3) that environmental variations modulate rRNA synthesis by repressing or derepressing specific gene clusters.