A novel Arabidopsis DNA binding protein contains the conserved motif of HMG-box proteins.

We have isolated cDNA clones from a lambda gtll library of mRNA prepared from rosette plants of Arabidopsis thaUana to identify DNA binding proteins that bind to the promoter region of the desiccation-responsive rd29A gene (1, 2, 3). We used a 40 bp conserved sequence (3) in the promoter regions of rd29A and rd29B genes, which may contain enhancer-like sequence(s), as a probe for the screening. We isolated two independent cDNA clones that bind to the 40-bp sequence. One of the cDNAs encodes a putative protein of 643 amino acids (aa) which contains a highly conserved sequence of high mobility group (HMG) protein which is referred to as ATHMG. The molecular weight of the putative ATHMG protein is 71,547. Yeast nonhistone chromosomal protein NHP6A (4) of 93 aa has the highest similarity with the carboxyterminal region of the ATHMG protein although it differs in size, as shown in Figure 1 A. The ATHMG protein contains the HMG box in the carboxyterminal region. In plants, two soybean cDNAs, SB11A (5) and SB16A (6), and one maize cDNA (7) have been reported to contain a HMGprotein like domain (Figure IB). However, their putative proteins are only 152, 153 and 157 aa long, respectively. Fewer sequence similarities were observed between the HMG-box domains in ATHMG and other plant HMG-like proteins (Figure IB). The ATHMG protein contains consensus motifs for the phosphorylation site of casein kinase II (Figure 1A) which is located in the upstream region of the HMG box. An acidic region is located upstream from the basic HMG box, while other HMG proteins contain the acidic regions in the carboxyterminal region. The remaining 500-aa amino terminal region of the ATHMG protein does not contain motifs or regions which are homologous with the reported proteins in the protein data base. The function of this long amino terminal region remains to be elucidated.