De novo synthesis and specific assembly of keratin filaments in nonepithelial cells after microinjection of mRNA for epidermal keratin. 
Poly(A)+ RNA isolated from bovine muzzle epidermis was microinjected into nonepithelial cells containing only intermediate-sized filaments of the vimentin type.  In recipient cells keratin polypeptides are synthesized and assemble into intermediate-sized filaments at multiple dispersed sites.  We describe the time course and the pattern of de novo assembly of keratin filaments within living cells.  These filaments were indistinguishable, by immunofluorescence and immunoelectron microscopic criteria, from keratin filament arrays present in true epithelial cells.  The presence of extended keratin fibril meshworks in these injected cells is compatible with cell growth and mitosis.  Double immunolabeling revealed that newly assembled keratin was not codistributed with microfilament bundles, microtubules or vimentin filaments.  We suggest that assembly mechanisms exist which in vivo sort out newly synthesized cytokeratin polypeptides from vimentin.