MOLECULAR-CLONING OF THE GENE ENCODING NUCLEAR-DNA HELICASE-II - A BOVINE HOMOLOG OF HUMAN RNA HELICASE-A AND DROSOPHILA MLE-PROTEIN

Nuclear DNA helicase II (NDH II) unwinds both DNA and RNA (Zhang, S., and Grosse, F. (1994) Biochemistry 33, 3906-3912). Here, we report on the molecular cloning and sequence determination of the complementary DNA (cDNA) coding for this DNA and RNA helicase. The full-length cDNA sequence was derived from overlapping clones that were detected by imm unoscreening of a calf thymus cDNA library in bacteriophage lambda gt1 1. This cDNA was 4,528 bases in length, which corresponded well with a 4.5-4.7-kilobase-long mRNA as detected by Northern blot analysis. The open reading frame of NDH II cDNA predicts a polypeptide of 1287 amin o acids and a calculated molecular mass of 141,854 daltons. NDH II is related to a group of nucleic acid helicases from the DEAD/H box famil y II, with the signature motif DEIH in domain II. Two further proteins of this family, i.e. human RNA helicase A and Drosophila Maleless (Ml e) protein, were found to be highly homologous to NDH II. With RNA hel icase A, there was 91.5% identity and 95.5% similarity between the ami no acid residues; with Mle protein, we observed a 50% identity and an 85% similarity. Antibodies against human RNA helicase A crossreacted w ith NDH II, further supporting that NDH II is the bovine homologue of human RNA helicase A. Immunofluorescence studies revealed a mainly nuc lear localization of NDH II. A role for NDH II in nuclear DNA and RNA metabolism is suggested.