Mouse cytokeratin EndoA is an intermediate filament subunit of the type II cytokeratin class which initiates expression in trophectoderm cells of blastocyst during embryogenesis. To identify the regulatory elements of the endo A gene, we constructed a series of CAT expression vectors and transfected them into PYS-2 cells. We found an enhancer element locating 1 kb downstream from the endo A gene which acts on both the endo A and SV40 promoters. This enhancer consists of six direct repeated sequences with homology to the PEA3 motif in polyoma virus alpha enhancer core. In undifferentiated F9 embryonal carcinoma cells, expression of the construct containing the enhancer was not detected. These results indicate that one of the regulatory mechanisms of endo A gene expression is the 3' downstream enhancer.

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It was reported that 080 lysogens did not adsorb superinf ecting phage X80 and a gene responsible to the phenomenon was named cor (Kozyrev et al., 1982). We found that E. coli containing X80 genome in lytic cycle also began to show resistance to superinf ecting ¢80 after 5 min of infection. We cloned the cor gene and determined its nucleotide sequence. A putative cor gene product should be 92 amino acids long and have three hydrophobic cores and one hydrophilic region, indicating a possibility of membrane binding protein. The product may interact with 080 receptor on inner-or outer-membrane of E. coli cells and inhibit the adsorption of superinf ecting 080. The cor gene is supposed to be transcribed from a unique promoter located close to the gene.

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