A pea homologue of human DNA helicase I is localized within the dense fibrillar component of the nucleolus and stimulated by phosphorylation with CK2and cdc2 protein kinases

DNA helicases catalyse the transient opening of duplex DNA during nucleic a cid transactions. Here we report the isolation of a second nuclear DNA heli case (65 kDa) from Pisum sativum (pea) designated pea DNA helicase 65 (PDH6 5). The enzyme was immunoaffinity purified using an antihuman DNA helicase I (HDH I) antibody column. The purified PDH65 showed ATP- and Mg2+-dependen t DNA and RNA unwinding activities, as well as ssDNA-dependent ATPase activ ity. The direction of DNA unwinding was 3' to 5' along the bound strand. An tibodies against HDH I recognized the purified PDH65, and immunodepletion w ith these antibodies removed the DNA and RNA unwinding and ATPase activitie s from purified preparations of PDH65. The DNA and RNA unwinding activities were upregulated after phosphorylation of PDH65 with CK2 and cdc2 protein kinases. By incorporation of BrUTP into pea root tissue, followed by double immunofluorescence labelling and confocal microscopy, PDH65 was shown to b e localized within the dense fibrillar component of pea root nucleoli in th e regions around the rDNA transcription sites. These observations suggest t hat PDH65 may be involved both in rDNA transcription and in the early stage s of pre-rRNA processing.