TFIIIA

TFIIIA, or transcription factor IIIA, is a eukaryotic transcription factor essential for transcription of 5S ribosomal RNA genes by RNA polymerase III. In vertebrates it is a single polypeptide containing nine C2H2-type zinc finger motifs, which provide sequence-specific DNA binding to the internal promoter elements of the 5S rRNA gene. TFIIIA functions as the initial DNA-binding factor that recognizes the internal promoter and serves as a platform for the assembly of the Pol III transcription apparatus.

TFIIIA binds to the internal promoter elements Box A and Box C within the transcribed region of the 5S rRNA gene and recruits TFIIIC, a multi-subunit complex. TFIIIC then recruits TFIIIB, which in turn positions RNA polymerase III for transcription initiation. The full assembly enables transcription of 5S rRNA genes by Pol III and is coordinated to ensure efficient transcription of these essential genes.

TFIIIA was first identified in Xenopus laevis oocytes as a factor required for Pol III transcription of 5S rRNA genes. It is conserved across eukaryotes; in humans the gene is known as GTF3A. Vertebrate TFIIIA is typically a single protein with nine zinc fingers, and homologs are found in plants and other animals, reflecting its central role in Pol III transcription of 5S rRNA.

In addition to its direct role in transcription, TFIIIA can influence chromatin structure around 5S rRNA gene repeats and may affect their copy number or accessibility in some systems. Its activity depends on proper interactions with TFIIIC and TFIIIB for productive transcription initiation by RNA polymerase III.