Dopa oxidase expression in fibroblast X melanoma fusions: Extinction in heterokaryons and maintenance in non-dividing cybrids

Pigmented B-16 mouse melanoma cells were fused with chick embryo fibroblasts or fibroblast cytoplasts and maintained as heterokaryons or non-dividing cybrids, respectively. These single cells were examined ultrastructurally for evidence of pigment gene expression using a cytochemical test for dopa oxidase, the initial enzyme in the conversion of dopa to melanin. Heterokaryons showed significantly less enzyme activity than control cells, whereas non-dividing cybrids showed no significant difference. Therefore, the presence of the intact nuclear membranes in the heterokaryons did not serve as a barrier to the interactions resulting in extinction of differentiated function(s). However, the presence of the fibroblast nucleus was necessary to elicit continued response.