The covalent attachment of the 76 amino acid protein ubiquitin is an important prerequisite for the degradation of many eukaryotic proteins. The specificity of this ligation is accomplished in part by a family of distinct ubiquitin conjugating enzymes (E2s) working in concert with specific ubiquitin-protein ligases (E3s). Three essential E2s in yeast encoded by ScUBC1, -4, and -5 comprise a functionally overlapping E2 subfamily that appears responsible for degrading most abnormal and short-lived proteins. A 15 kDa E2 protein homologous to this family has been identified previously in wheat germ, designated TaE2(15kDa) (Girod and Vierstra (1993) J. Biol. Chem. 268, 955-960). This E2 is responsible for much of the ubiquitin conjugating activity observed in wheat germ extracts and works together with a unique E3 (designated E3 gamma) for substrate recognition. In this paper, the cloning of five genes encoding E2(15kDa) from Arabidiopsis thaliana is described (designated AtUBC8-12). They encode 149 amino acid basic proteins 94-98% similar to each other and 88-92% similar to ScUBC4 at the amino acid sequence level. In contrast, AtUBC8-12 are only 55-65% similar to the Arabidopsis E2s encoded by AtUBC1, -4, and -7. The AtUBC8-12 proteins do not contain N- or C-terminal extensions and have the active site at residue Cys-86, based on their homology with other E2s. Analyses of genomic Southern blots are consistent with the existence of multiple members encoding this E2 subfamily. AtUBC8-12 are transcribed to yield about 800 nucleotide mRNAs that, unlike ScUBC4 and -5, are not strongly induced by heat shock.(ABSTRACT TRUNCATED AT 250 WORDS)