The PROLIFERA (PRL) gene in Arabidopsis encodes a homologue of the yeast and mammalian MCM proteins which are responsible for regulating the initiation of DNA replication

The PROLIFERA (PRL) gene in Arabidopsis encodes a homologue of the yeast and mammalian MCM proteins which are responsible for regulating the initiation of DNA replication (Springer et al., 1995). [It should be noted that the gene PLEIOTROPIC REGULATOR LOCUS in Arabidopsis, details of which were recently published by Németh et al. (1998), has unfortunately also been given the abbreviation ‘PRL’. This gene is unrelated to PROLIFERA (Springer et al., 1995)]. There are 6 MCM proteins (Mcm2-7) that form one or more protein complexes that enter the nucleus, bind chromatin and regulate replication during G1. Triggering of replication is accomplished by displacement of the protein complex from chromatin, which in turn is associated with Cdc6-dependent phosphorylation of the MCM protein complex (reviewed by Kearsey and Labib, 1998). In yeast, MCM proteins regulate cell division by nuclear uptake and export, while in mammalian cells, chromatin binding is regulated. PROLIFERA is a homologue of Cdc47 (Mcm7) which is taken up by the nucleus during the G1 phase of the yeast cell cycle (Dalton and Whitbread, 1995). The CDC47 gene is tightly transcriptionally regulated in yeast, being highly expressed only at the M/G1 boundary. Protein levels remain constant throughout the cell cycle, however the subcellular localization pattern changes as the protein is rapidly exported from the nucleus during the S phase, rather than being degraded (Dalton and Whitbread, 1995). Cdc47 functions during S phase, being required for the initiation of DNA replication along with ORC (Origin Recognition Complex), Cdc6 and the other members of the MCM family. At the restrictive temperature, mutant cdc47 yeast cells arrest at a variety of stages during S phase, perhaps reflecting the firing of Cdc47-independent origins of replication. In the embryo and endosperm following fertilization, PROLIFERA was found to be expressed from both paternally and maternally transmitted alleles, although a GUS gene trap fusion protein was only detected when it was transmitted through the female. It is therefore possible that much of the PROLIFERA protein is maternally accumulated, consistent with the larger nuclear volume contributed to the zygote and especially to the central cell by the female gametophyte. 1815 Development 127, 1815-1822 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV0284