Essential protein (PubMed:22353599). Isoform 1 is a transcription activator the binds both 5S rDNA and 5S rRNA and stimulates the transcription of 5S rRNA gene (PubMed:12711688, PubMed:22353599). Isoform 1 regulates 5S rRNA levels during development (PubMed:22353599). Accumulates in several nuclear foci including the nucleolus. Expressed in seedlings, flowers, siliques and seeds. Both isoforms 1 and 2 transcripts are expressed in seedlings and both TFIIIA and TFIIIA-C are present at similar ratios in 10-day-old seedlings (at protein level). In correlation with the reorganization of 5S rDNA chromatin to a mature state, full-length isoform 1 protein TFIIIA with transcriptional activity accumulates and permits de novo transcription of 5S rRNA. Isoforms 1 and 2 transcripts accumulate in various tissues of the reproductive phase, including flowers and siliques, but only isoform 2 is present in mature seeds. In flowers, both TFIIIA and TFIIIA-C are present at similar ratios (at protein level). Very low amounts of TFIIIA are found in extracts from fresh siliques compared to TFIIIA-C. The amount of full-length TFIIIA protein progressively increases from fresh siliques to seeds concomitant with lower proportions of the shorter TFIIIA-C protein (at protein level) thus leading to 5S rRNA accumulation in the seed. Protein product TFIIIA (44 kDa) is proteolytically cleaved into TFIIIA-C (34 kDa). Lethal. Binds in vitro potato spindle tuber viroid (PSTVd) RNA. Degraded through the nonsense-mediated mRNA decay (NMD) pathway in a regulated unproductive splicing and translation (RUST) process.