The genes coding for ribosomal proteins (r-proteins) in Xenopus laevis have a stretch of about 20 pyrimidines (Y) at their 5' end, in the middle of which is localized the main transcription start point (tsp). To obtain information about its possible functional significance, we have introduced by site-directed mutagenesis one or more purines at various positions within the oligo(Y) tract present at the 5' end of the gene coding for r-protein S19 of X. laevis. The effect of these mutations on transcription and translation of a reporter-coding sequence has been evaluated by DNA microinjection in X. laevis oocytes. We show that an uninterrupted stretch of pyrimidines is not necessary for efficient transcription and translation of the reporter gene in the X. laevis oocyte. This finding does not exclude the possibility that this sequence is involved in transcription and/or translation regulation in some developmental situations different from oogenesis.