Evidence for an abundant 33,000-dalton polypeptide regulated by cytokinins in cultured tobacco tissues. 
When cloned pith and leaf tissues of Nicotiana tabacum L. cv.  Havana 425 are subcultured for 3 d on auxin-containing medium and labelled for 18 h with [(35)S]methionine, up to 10% of the labelled, soluble-protein fraction is found in a single band with an apparent molecular weight of approx.  32,000-34,000 dalton on sodium-dodecylsulfate polyacrylamide-gel electrophoretograms.  The labelling of this band, designated P33, is dramatically inhibited by the cytokinin, kinetin, in some cell lines at concentrations as low as 1.4·10(-8) M. P33 is a major component of the protein fraction obtained from non-habituated clones, cytokinin-habituated clones, and revertant subclones of crown-gall-transformed clones, but cannot be detected in clones habituated for both auxin and cytokinin, or crown-gall-transformed clones.  The evidence supports the hypothesis that cytokinin in the presence of auxin regulates the production of a specific, major polypeptide in the soluble-protein fraction of the tissue and that this protein is not produced in tissues autotrophic for both auxin and cytokinin.