The binding sites for Xenopus laevis FIII/YY1 in the first exon of L1 and L14 ribosomal protein genes are dispensable for promoter expression

The genes coding for the ribosomal proteins (rp genes) L14 and L1 in the toad Xenopus laevis are contacted in the first exon by the frog protein, FIII/YY1, homolog of the human zinc-finger protein YY1, acting as repressor, activator and initiator of transcription. To investigate the functional significance of FIII/YY1 in the context of the two rp genes, the L14 region at nucleotide positions -105 to +44, including all of the first exon was linked to the chloramphenicol acetyltransferase (CAT) reporter gene; constructs with wild-type and mutated sites for FIII/YY1 were injected into nuclei of stage V-VI oocytes and analyzed for CAT activity. The same procedure was followed for constructs made with L1 sequences at nucleotide positions -17 to +1567. Mutations in the sites for FIII/YY1 did not change reporter activity, nor did overexpression of FIII/YY1 in the oocytes prior to injection with L1 and L14 constructs. Since oocytes are non-dividing cells, transfections were made of Xenopus kidney cells in culture with the same constructs and the results obtained in oocytes confirmed.