ELSEVIER Biochimica et Biophysica Acta 1305 (1996) 109-112 Biochi f ic~a etBiophysica A~ta Short sequence-paper Cloning and characterization of two transcripts generated from the mel-13 gene positioned adjacent to the mammalian Polycomb group-related gene mel-18 Osamu Tetsu a.b, Rieko Kanno a, Kaichi Isono b, Masaru Taniguchi a, Masamoto Kanno a.* Division of Molecular Immunology, Center for Biomedical Science, Chiba Universi~', 1-8-1 lnohana, Chuo-ku, Chiba, 260 Japan b Department of Surgery, School of Medicine, Chiba Universi~,, 1-8-1 lnohana, Chuo-ku, Chiba, 260 Japan Received 20 September 1995; revised 13 November 1995; accepted 22 November 1995 Abstract We previously isolated the mel-18 gene, a mammalian Polycomb group (PcG)-related gene with homology to bmi-1 oncogene. We show in this paper the existence of a new gene, mel-13, which overlapped with the mel-18 anti-oncogene. We discuss the relationships between mel-13 and the mel-18, bup, and Su(z)2 genes. Keywords: Mel-18; cDNA cloning; Genomic structure; Alternative splicing; Overlapping gene The mel-18/bmi-1 gene family is considered to be a mammalian homologue of Drosophila Polycomb group (PcG) genes, and shows especially high homology to Posterior sex combs (Psc) gene [1-4]. The Psc gene is flanked by the Suppressor two of zeste, Su(z)2, gene, which encodes a protein that shares a 200 amino acid domain with 37% sequence identity with Psc [5]. The bmi-1 oncogene is also known to be flanked by a gene called bup, although the bup gene, whose function is unknown, shows no homology to any Su(z)2 or PcG genes of Drosophila so far examined [6]. Here we have identi- fied a new gene, mel-13, flanked by the mel-18 anti- oncogene and analyzed the relationships with the reel-18, bup, and Su(z)2 genes. We have detected a transcript corresponding to 13S RNA hybridized to a genomic DNA probe downstream of the mel-18 gene. Since mel-18 cDNA did not detect any bands corresponding to the 13S RNA, we speculated that another gene was also located in this area [7]. Therefore, we screened a hgtl0 cDNA library of B16 mouse melanoma with the same genomic probe and identified four cDNA clones, clones 1, 5, 20, and 101. All these cDNAs give a 13S band in Northern blot analysis. Interest- * Corresponding author. Fax: + 81 43 2271498; e-mail: kanno @med.m.chiba-u.ac.jp. 0167-4781/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 67-478 1 (95)00229-4 ingly, clone 5 and clone 101 have identical nucleotide sequences downstream from nucleotide position 304 (clone 101 numbering), but the 5' parts of the cDNAs differ, implying alternative splicing mechanisms (Fig. 2). The cDNA sequences of clones 1 and 20 contain a shared region with clones 5 and 101 and more extended 3' regions. We designated the two different cDNA forms as mel-13a and mel-13b, respectively (Fig. 2A). The nucleotide sequences of these cDNAs and the deduced amino acids sequences are shown in Fig. 2B. The mel-13a and mel-13b cDNAs consist of 865 and 1071 bp and the longest open reading frames are translated into 132 and 73 amino acids with deduced molecular masses of 14.9 and 7.8 kDa, respectively. There is a polyadenylation signal at nucleotide position 830. There are two possible protein kinase C phosphorylation sites, one cAMP depen- dent phosphorylation site, and two myristylation sites on the Mel-13a protein. There are two possible myristylation sites on the Mel-13b protein. The nucleotide and amino acid sequences were examined for homology with the recent GenBank/EMBL and PIR/Swiss-Prot databases using the BLAST or MP search programs with Smith- Waterman algorithm. We found several following homolo- gies with mel-13a protein; (i) Ras-related protein RAC1 which belongs to the GTP binding Rho sub-family (at amino acid position 4 to 21 of Mel-13a with 60% match), (ii) energy-dependent vesicular-mediated transport protein