SPECIALIZED FORM OF RNA-POLYMERASE-I, ESSENTIAL FOR INITIATION AND GROWTH-DEPENDENT REGULATION OF RIBOSOMAL-RNA SYNTHESIS, IS DISRUPTED DURING TRANSCRIPTION

Only a small proportion (<2%) of RNA polymerase I (pol I) from whole-c ell extracts appeared to be competent for specific initiation at the r ibosomal gene promoter in a yeast reconstituted transcription system. Initiation-competent pol I molecules were found exclusively in salt-re sistant complexes that contain the pol I-specific initiation factor Rr n3p. Levels of initiation-competent complexes in extracts were indepen dent of total Rrn3p content and varied with the growth state of the ce lls. Although extracts from stationary phase cells contained substanti al amounts of Rrn3p and pol I, they lacked the pol I-Rrn3p complex and were inactive in promoter-dependent transcription. Activity was resto red by adding purified pol I-Rrn3p complex to extracts from stationary phase cells. The pol I-Rrn3p complex dissociated during transcription and lost its capacity for subsequent reinitiation in vitro, suggestin g a stoichiometric rather than a catalytic activity in initiation. me propose that the formation and disruption of the pol I-Rrn3p complex r eflects a molecular switch for regulating rRNA synthesis and its growt h rate-dependent regulation.