Papers in the Biological Sciences


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Nucleic Acids Research, 2003, Vol. 31, No. 2 619±628


Copyright 2003 Oxford University Press


The finding in animal species of complexes homologous

to the products of six Saccharomyces cerevisiae

genes, origin of replication recognition

complex (ORC), has suggested that ORC-related

mechanisms have been conserved in all eukaryotes.

In plants, however, the only cloned putative homologs

of ORC subunits are the Arabidopsis ORC2 and

the rice ORC1. Homologs of other subunits of plant

origin have not been cloned and characterized. A

striking observation was the absence from the

Arabidopsis genome of an obvious candidate genehomolog

of ORC4. This fact raised compelling

questions of whether plants, in general, and

Arabidopsis, in particular, may have lost the ORC4

gene, whether ORC-homologous subunits function

within a complex in plants, whether an ORC complex

may form and function without an ORC4 subunit,

whether a functional (but not sequence)

protein homolog may have taken up the role of

ORC4 in Arabidopsis, and whether lack of ORC4 is a

plant feature, in general. Here, we report the first

cloned and molecularly characterized five genes

coding for the maize putative homologs of ORC subunits

ZmORC1, ZmORC2, ZmORC3, ZmORC4 and

ZmORC5. Their expression proiles in tissues with

different cell-dividing activities are compatible with

a role in DNA replication. Based on the potential of

ORC-homologous maize proteins to bind each other

in yeast, we propose a model for their possible

assembly within a maize ORC. The isolation and

molecular characterization of an ORC4-homologous

gene from maize argues that, in its evolution,

Arabidopsis may have lost the homologous ORC4


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