42SP48 IN PREVITELLOGENIC XENOPUS OOCYTES IS STRUCTURALLY HOMOLOGOUS TO EF-1-ALPHA AND MAY BE A STAGE-SPECIFIC ELONGATION-FACTOR

We have isolated the cDNA for 42Sp48 and EF-1-alpha from mixed stage oocytes and tailbud (stage 22) Xenopus laevis cDNA libraries by use of the cDNA for human elongation factor-1-alpha (EF-1-alpha) as probe. The nucleotide and deduced amino acid sequences of the entire coding region of 42Sp48 and EF-1-alpha cDNA were established. The proposed functional homology of the proteins is reflected in highly conserved amino acid sequences (91% identity), while the large number of silent mutations at the gene level may serve to prevent recombination at their loci. 42Sp48 is apparently encoded by two genes in Xenopus, while no sequences corresponding to 42Sp48 could be found in murine or human genomic DNA. 42Sp48 has been proposed to act as a stage-specific elongation factor in Xenopus. Comparison of the deducted amino acid sequences of 42Sp48 and EF-1-alpha with that of elongation factor Tu from E. coli, for which the three-dimensional structure including that of the GTP binding site have been determined, supports this hypothesis.