Synthesis of eukaryotic 5S rRNA:

In the eukaryotes,  the  5S  rRNA  gene  is  also  present  in  multiple  copies  (2000  in human  cells and  all  clustered  together  at  one  chromosomal   site).  Unlike other eukaryotic rRNA genes, the 5S rRNA genes are transcribed through RNA polymerase III (RNA Pol III). The promoters of tRNA genes that are also transcribed through RNA Pol III contain control elements known as the A box and B box situated down- stream of the transcriptional begin site. A same condition exists for 5S rRNA genes in which the promoter has two control elements located down- stream of the transcriptional  begin site, an A box and a C box shown in the figure. The C box binds  transcription   factor  IIIA  (TFIIIA)  that  then  in  turn  interacts  with TFIIIC to cause it to bind, a procedure that possibly also included recognition of the A box. One time TFIIIC has bound, TFIIIB binds and interacts with RNA Pol III causing which to bind also to form the transcription initiation complex.  One of the three units is TATA box binding protein TBP, the transcription factor of TFIIB needed for transcription through all three eukaryotic RNA polymerases. Subsequent   transcription, the 5S rRNA   transcript   needs   no processing it migrates to the nucleolus and is recruited into ribosome assembly.

Figure:   Initiation of transcription of a 5S rRNA gene by RNA Pol III.