PZF, a cDNA Isolated from Lotus japonicus and Soybean Root Nodule Libraries, Encodes a New Plant Member of the RING-Finger Family of Zinc-Binding Proteins

Low-stringency screening of a soybean root nodule cDNA library (106 recombinants screened) with a probe encoding the bovine rasGAP catalytic domain (Marshall et al., 1989) permitted the isolation of a rare cDNA that contains a 500-bp region of 68% similarity to the rasGAP cDNA. This partia1 soybean cDNA was used to isolate a full-length cDNA of 2.1 kb from the root nodule cDNA library of the model legume Lotus japonicus (Handberg and Stougaard, 1992). The full-length cDNA contains only one long ORF coding for a translation product of 60 kD. Searching the GenBank and EMBL data bases with FASTA (Pearson and Lipman, 1989) and BLAST (Altschul et al., 1990) revealed only little overall similarity to known proteins. The most prominent characteristic is the presence of a putative C3HC4 Zn-finger domain, the RING-finger domain (Lovering et al., 1993), suggesting that the plant proteins deduced from the cDNA sequences have a regulatory function. Consequently, the soybean and L. japonicus cDNAs were designated Gmpzf and Ljpzf, respectively. The deduced proteins were named Pzf (Table I). Most RING-finger family members appear to be regulatory transcription factors or DNA handling tools. The group comprises severa1 mammalian oncoproteins, homeotic gene products in Drosopkila, and DNA repair proteins in yeast and mammals. Furthermore, many members are encoded by immediate-early genes of a variety of viruses (Lovering et al., 1993). Biochemical evidence for Zn2+ and DNA binding due to the C3HC4 domain was established for some members, such as Me118 (Tagawa et al., 1990), RINGl (Lovering et al., 1993), and COPl (von Arnim and Deng, 1993). The only plant RING-finger family member identified so far is AtCOPl from Arabidopsis tkaliana (Deng et al., 1992). This putative transcription factor also possesses homology to p subunits of heterotrimeric G proteins. Thus, COPl may link phytochrome-mediated signal reception directly to regulation of transcriptional activities. The Ljpzf protein sequence contains an upstream HKRK motif (aa 141-145) that might serve as a NLS. The protein also contains other regions of clustered basic residues that might be involved in nucleic acid binding. The protein is