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A major site of DNA bending is located 1.6 kb upstream of the P1 transcription start site of the human c-myc gene, near the center of a reported zone of initiation of DNA replication. A repeated, purine-rich element, termed PUR, at the bend site is specifically bound by a protein in HeLa cell nuclear extracts. This protein has specific affinity for the purine-rich single strand of the element. Methylation interference maps a pattern of specific contact points with guanosine bases in a 24-mer oligonucleotide containing the element. UV cross-linking reveals that contact is made by a polypeptide of approximately 28 kDa. The PUR element is present at origins of replication and in gene flanking regions in a variety of eukaryotes from yeasts through humans. The consensus sequence GGNNGAGGGAGARRRR has been derived. This element is present near centers of regions of two mammalian loci (human c-myc and hamster dhfr) recently reported as initiation zones for DNA replication. A 24-mer oligonucleotide representing the hamster dhfr version of the PUR element effectively competes with the human c-myc version for binding to Pur.It is not known at this time whether cis-acting elements serve to control the initiation of DNA replication in mammalian chromosomes. Furthermore, no mammalian protein has been shown to influence the initiation of replication through sequence-specific binding to DNA. It is, however, clear that initiation of replication in prokaryotes, lower eukaryotes, and mammalian DNA tumor viruses involves duplex opening mediated by the specific binding of an initiator protein to repeated DNA elements (for reviews, see references 6, 20, 24, and 48). Difficulty in identifying sequences controlling mammalian initiation has been due in part to a lack of suitable methods for mapping replication origins in large and complex genomes. Recent mapping procedures have improved the localization of initiation zones in the vicinity of certain mammalian genes, and at least two candidates for potential origins exist. Seven different mapping methods concur in locating an initiation zone approximately 17 kb 3' to the hamster dihydrofolate reductase

Using genetic inactivation in the mouse, PURA, encoding Pur alpha, is demonstrated to be essential for developmentally-timed dendrite formation in the cerebellum and hippocampus. Comparison of RNA species bound by Pur alpha prompts the hypothesis that Pur alpha functions with non-coding RNA in transport of certain mRNA molecules to sites of translation in dendrites. Pur alpha binds to human BC200 RNA, implicated in dendritic targeting, and this has homologies to 7SL RNA, implicated in compartmentalized translation. Results using hippocampal rat neurons in situ show that Pur alpha binds to BC1 RNA, implicated in dendritic targeting as a mouse counterpart of BC200, and to mRNA molecules translated in dendrites; Pur alpha is specifically located in dendrites, where it is colocalized with Map2, but not in axons, where it fails to colocalize with Ankyrin G. Pur alpha and Staufen are colocalized at dendritic sites of mRNA translation. Microtubule disruptors inhibit Pur alpha dendritic targeting and allow its mislocalization to axons. Using mouse brain, double-RNA immunoprecipitation places Pur alpha together with Staufen or FMRP on BC1 RNA and specific mRNA species in vivo. These results help define a mechanism by which Pur alpha targets specific mRNA molecules to sites of dendritic translation.

Puralpha is a ubiquitous, sequence-specific DNA- and RNA-binding protein which is highly conserved in eukaryotic cells. Puralpha has been implicated in diverse cellular functions, including transcriptional activation and repression, translation and cell growth. Moreover, this protein has been shown to be involved in regulating several human viruses which replicate in the central nervous system (CNS), including human immunodeficiency virus type I (HIV-1) and JC virus (JCV). Puralpha exerts part of its activity by interacting with cellular proteins, including pRb, E2F, cyclin A, Sp1 and members of the Y-box family of proteins, including YB-1 and MSY1, as well as viral proteins such as polyomavirus large T-antigen and HIV-1 Tat. The ability of Puralpha to interact with its target DNA sequence and to associate with several viral and cellular proteins is modulated by RNA. Puralpha has also been shown to be involved in cell growth and proliferation. Its association with pRb, E2F and cyclin A coupled with its fluctuating levels throughout the cell cycle, position Puralpha as a crucial factor in the cell cycle. Moreover, microinjection studies demonstrate that Puralpha causes either a G(1) or G(2) arrest depending on the cell cycle time of injection. The gene encoding Puralpha has been localized to a human locus which is frequently deleted in myelogenous leukemias and other cancers and Puralpha gene deletions have been detected in many cases of lymphoid cancers. The following review details the structural characteristics of Puralpha, its family members and the involvement of this protein in regulating various cellular and viral genes, viral replication and cell growth.

The human Pur factor binds strongly to a sequence element repeated within zones of initiation of DNA replication in several eukaryotic cells.