Orc Gene Family In Bombyx Mori

The heterohexameric DNA replication origin recognition complex (ORC) had been separated from yeast originally, and named as Orc1p-6p according to the molecular weight. Now it has been found in all the eukaryotes examined. ORC binds to the origin of DNA replication and acts as a landing pad for the recruitment of other replication factors that form the pre-replication complex (pre-RC). Individual subunits of ORC have additional functions that are unrelated to DNA replication:Orc1p mediates transcriptional silencing; Orc2p,3p,4p and 5p (core complex) promote the dendrite development in the central nervous system; and Orc6p participates in cytokinesis. In lepidoptera, only the gene for the sixth ORC subunits, ORC6 from the moth Choristoneura fumiferana, has been cloned previously.The cDNA sequences and the genomic sequences of six subunits of ORC in Bombyx mori (BmORC1-6) were determined by RACE (Rapid Amplification of cDNA Ends) and bioinformatics analysis. Phylogenetic analysis of the predicted amino acid sequences of BmOrc1p-6p revealed that they were more similar to their corresponding counterparts from other organisms including chordates and yeast than to each other. During embryo development, the six BmORC genes were regulated coordinately, but different ratios of the abundances of mRNA were observed in thirteen tissues of the 6th day of 5th-instar larvae. Infection by B. mori nucleopolyhedrovirus (BmNPV) initially decreased and then increased the expression of BmORC at mRNA level until 36th hour.The six protein subunits were named BmOrc1p-6p, respectively. The amino terminal of BmOrc1p was incomplete. BmOrc1p-5p contained weakly conserved AAA+ domains. The carboxyl terminal of BmOrc6p contained a short fragment which was only discovered in lepidoptera presently. The greater similarity between individual ORC proteins from different organisms than the similarity between BmOrc1p-6p indicated that each subunit of BmOrc proteins evolved individually from the time before the phylogenetic separation of yeast, chordates and insects. The similar expression profiles of BmORCs indicated that they functioned as a heteromeric complex during the embryo development. However, the ratios between BmORC mRNAs in different tissues of the 5th instar larva varied greatly, indicating that the some of the BmORC proteins may have individual functions independent of the heterohexameric complex. The coordinate changes in the expression of all six BmORC mRNAs suggested that the BmORC proteins might also participate in the replication of B. mori nucleopolyhedrovirus (BmNPV).