Worm Breeder's Gazette 11(2): 51
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have identified an embryonic nuclear antigen that has several similarities to the Xenopus histone-binding protein N1 (see Kleinschmidt and Seiter, EMBO 7:1605,1988). The antigen was identified in a screen of C. elegans expression libraries with sera from rabbits immunized with homogenates of early embryos. Plaques of positive clones were used to affinity purify cognate antibodies from the sera, which were then assayed for staining of fixed embryos by indirect immunofluorescence microscopy (Burkholder and Wood, C. elegans meeting abstr. 1989). We obtained an antibody preparation that recognizes a diffuse nuclear antigen, present in oocytes beginning at pachytene and in all nuclei of early embryos, but becoming more restricted later in development. In larvae, the staining is less intense and only in subsets of nuclei that correlate with the positions of proliferating cells, and in adults only maturing oocytes contain the antigen. In mitotic cells the antigen is diffuse throughout the cytoplasm, and then appears to reassemble into the nucleus neither associated with chromatin or the nuclear envelope. We have studied the cloned gene encoding this nuclear antigen, which is named D1. The gene encodes a 2kb messenger RNA which is relatively abundant in early embryos, apparently resulting from the accumulation of maternally supplied transcripts. Late embryos and L1's have very low levels of message. The message encodes a 583 a.a. protein with a predicted MW of 63kd, although the antibody detects a protein of 1 00kd on an immunoblot of embryonic proteins. This discrepancy is likely due to the highly charged nature of the protein, which has 25% acidic residues including one region where 13/18 residues are negatively charged. A highly basic region near the C-terminus ( including the sequence TKKRKS) may function as the nuclear localization signal. In a search of the Genbank protein database the most homologous protein was the Xenopus N1 protein, although sequence identity between the two proteins is only 20% over 572 residues. This level of homology alone is not very convincing, but combined with the similar size, arrangement of highly charged regions, cellular location and developmental expression of the two proteins, we are exploring the possibility that the C. elegans D1 protein has a histone-binding function similar to the Xenopus N1 protein. Maternal stores of histones H3 and H4 are found in a complex with N1 protein, which is thought to promote chromatin formation during the rapid cleavage cycles of Xenopus embryos. The D1 gene maps to YAC Y11G11 on V (A. Coulson and J. Sulston, pers. comm.), which places it on LGV between myo-3 and rrs-1 on the genetic map. In that interval the only obvious candidate for a mutation in the D1 gene is the maternal-effect lethal mutation par-1; however, par-1 probably maps to the right of D1 according to physical mapping data (A. Telfer, pers. comm.).