A full-length cDNA encoding a mitochondrial DNA-specific single-stranded DNA binding protein fromXenopus laevis

We have isolated a full-length cDNA specifying the precursor of one of the two known single-stranded mitochondrial DNA (mtDNA) binding proteins of Xenopus laevis. The protein is called mtSSB-1 (1). The putative function of these proteins is to stabilize the displaced single strand of normal and expanded displacement loop (D-loop) during mtDNA replication (1, 2). We identified a single hybridizing clone, called XL31, from the screening of about 3 x 10 pfu of a library of Xenopus laevis ovary cell cDNA inserted into phage lambda UniZAP XR. The full-length 753-bp cDNA from XL31 includes: 1) a 158-nt GCrich (58%) 5' untranslated region; 2) a 51-nt region encoding a deduced 17-aa N-terminal presequence rich in basic amino acids (4/17, or 23%), and containing no acidic residues; 3) a 390-nt region encoding the deduced 129 aa-long mature polypeptide, plus a TGA stop codon; the deduced amino-terminal sequence of the mature protein is identical to the published N-terminus of mtSSB-1 (1); 4) a 154-nt 3' untranslated region, containing an AATAAA polyadenylation signal, located 20 bp upstream from a poly (A) tail. We assume that the polypeptide encoded by the cDNA starts at the ATG at nt 159, because: (i) it is the first ATG encountered in the cDNA, (ii) it is in a moderately favorable context for translation initiation (3), with a purine at position —3 relative to the A of ATG, and (iii) there are two in-frame potential stop codons upstream from the ATG at nt 159. Based on available data on the polypeptide sequences for the Xenopus laevis mtSSBs (1), the mature protein is assumed to start at the +18 serine residue.