COORDINATE REGULATION OF RIBOSOMAL COMPONENT SYNTHESIS IN ACANTHAMOEBA-CASTELLANII - 5S RNA-TRANSCRIPTION IS DOWN-REGULATED DURING ENCYSTMENT BY ALTERATION OF TFIIIA ACTIVITY

Transcription of large rRNA precursor and 5S RNA were examined during encystment of Acanthamoeba castellanii. Both transcription units are d own regulated almost coordinately during this process, though 5S RNA t ranscription is not as completely shut down as rRNA transcription. The protein components necessary for transcription of 5S RNA and tRNA wer e determined, and fractions containing transcription factors comparabl e to TFIIIA, TFIIIB, and TFIIIC, as well as RNA polymerase III and a 3 '-end processing activity, were identified. Regulation of 5S RNA trans cription could be recapitulated in vitro, and the activities of the re quired components were compared. In contrast to regulation of precurso r rRNA, there is no apparent change during encystment in the activity of the polymerase dedicated to 5S RNA expression. Similarly, the trans criptional and promoter-binding activities of TFIIIC are not altered i n parallel with 5S RNA regulation. TFIIIB transcriptional activity is unaltered in encysting cells. In contrast, both the transcriptional an d DNA-binding activities of TPIIIA are strongly reduced in nuclear ext racts from transcriptionally inactive cells. These results were analyz e? in terms of mechanisms for coordinate regulation of rRNA and 5S RNA expression.