3 DIFFERENT GENES ENCODE NM23 NUCLEOSIDE DIPHOSPHATE KINASES IN XENOPUS-LAEVIS

Nucleoside diphosphate kinases (NDPKs) catalyse the phosphorylation of nucleoside diphosphates. In mammals, the functional enzyme is a hexam er composed of different amounts of two homologous acidic (A) and basi c (B) subunits encoded by separate genes. In prokaryotes and invertebr ate eukaryotes, only one cytoplasmic enzyme has been isolated. Other g enes encoding chloroplastic and mitochondrial forms as well as related proteins have been cloned. Here, we show that in Xenopus laevis, as i n mammals, the cytoplasmic NDPK is encoded by several homologous genes . With Xenopus laevis being a pseudotetraploid species, each monomer i s encoded by two genes. The amino acid sequences are very similar, and all the differences concern amino acids located at the outer surface of the hexameric enzyme. The Xenopus genes share 82-87% identity with their human counterparts. Interestingly, in vitro, the Xenopus X1 enzy me binds to a specific nuclease hypersensitive element (NHE) of the hu man c-myc promoter, as does its human counterpart. X1 also binds to a single-stranded (CT)(n) dinucleotide repeat. The NHE is present in the coding strand of a pyrimidine-rich region of the 3' non-coding sequen ce of the Xenopus NDPK genes. We propose that NDPK is indeed able to b ind to its own mRNA and prevent polyadenylation at the normal position . This could provide an autoregulatory translation mechanism. A phylog enetic tree of the vertebrate NDPK sequences supports the idea that in amphibians, as in mammals, gene duplication has resulted in functiona l diversification. (C) 1997 Elsevier Science B.V.