Molecular cloning and characterization of MMS2, a yeast homolog of human CROC1. S. Broomfield, and W. Xiao, Department of Microbiology, University of Saskatchewan, 107 Wiggins Rd., Saskatoon, SK S7N 5E5, Canada. Methyl methanesulfonate (MMS) is an alkylating agent that donates methyl groups to DNA,, creating lesions, one of which, 3-methyladenine, is able to block DNA replication and is considered a major lethal lesion. A yeast mutant ( mms2-1) with an increased sensitivity to MMS, but not to UV or x-rays was isolated by Prakash and Prakash (Genetics 86, 1977). We have cloned and determined the sequence of the MMS2 gene by functional complementation of the MMS sensitive phenotype of the mms2-1 mutant. The MMS2 open reading frame encodes a 100 amino acid protein with a calculated molecular weight of 11.4 kDa. MMS2 is highly homologous to a family of human gene products, CROC-1A and CROC-1B which bind to the c-fos promoter enhancer sequence and likely act as transcriptional activators. MMS2 also shares significant sequence homology with ubiquitin conjugating proteins (UBCs). Studies to date have shown that mms2*delta* is sensitive to MMS, but not to other DNA damaging agents tested and that MMS2 mRNA is induced by heat shock treatment, but not by treatments with DNA damaging agents. We are testing functional cross complementation of MMS2 and CROC-1 in both yeast and human cultures.