Relative amounts of basic nuclear proteins SP4 and SP5 in Xenopus laevis sperm correlate with gene copy number

Mature sperm of an anuran amphibian Xenopus laevis contain six sperm-specific basic nuclear proteins (SP 1-6). In an attempt to understand how the amounts of SP produced are regulated during transcription and translation, the relative amounts of proteins and messenger RNA (mRNA), and the gene number in nuclei were compared between SP4 and SP5. Measurements of the peak areas of proteins separated by reversed-phase high performance liquid chromatography (HPLC) indicated that SP4 is present in about a five times greater amount than SP5. Nuclease S1 protection assays showed the existence of SP4 mRNA in about a five times greater amount than SP5 mRNA. Southern hybridization analyses of restriction enzyme-digested genomic DNA indicated a single copy of SP5 gene in the haploid genome, as contrasted with the five SP4 genes found in our previous study. Thus the deposition of different amounts of SP4 and SP5 in sperm nuclei reflects the relative number of these genes in the genome.