In eubacteria, PriA helicase detects the stalled DNA replication forks. This critical role of PriA is ascribed to its ability to bind to the 3 0 end of a nascent leading DNA strand in the stalled replication forks. The crystal structures in complexes with oligonucleotides and the combination of fluorescence correlation spectroscopy and mutagenesis reveal that the N-terminal domain of PriA possesses a binding pocket for the 3 0 -terminal nucleotide residue of DNA. The interaction with the deoxyribose 3 0 -OH is essential for the 3 0 -terminal recognition. In contrast, the direct interaction with 3 0 -end nucleobase is unexpected, considering the same affinity for oligonucleotides carrying the four bases at the 3 0 end. Thus, the N-terminal domain of PriA recognizes the 3 0 -end base in a base-non-selective manner, in addition to the deoxyribose and 5 0 -side phosphodiester group, of the 3 0 -terminal nucleotide to acquire both sufficient affinity and non-selectivity to find all of the stalled replication forks generated during DNA duplication. This unique feature is prerequisite for the proper positioning of the helicase domain of PriA on the unreplicated double-stranded DNA.

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We previously reported the presence of a new gene (HSET) with an unknown function, in the centromeric side of the class II gene region of the human major histocompatibility complex (MHC). cDNA clones corresponding to the HSET gene were isolated from a human testis cDNA library. A 2.4 kilobase transcript from the HSET gene was abundantly expressed in testis, B-cell, T-cell, and ovary cell lines but was not detected in lung or stomach. Analysis of the nucleotide sequence of the HSET cDNA clones revealed significant similarity to kinesin-related proteins in yeast, Drosophila, and human. Its predicted amino acid sequence contains a domain with strong sequence similarity to the ATP-binding and motor domains of a plus end-directed microtubule motor protein, kinesin, which might be involved in mitotic chromosome segregation, suggesting that the HSET gene encodes a novel kinesin-related protein.

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