We have characterized a mutant of avian myeloblastosis virus (strain GA907/7) that shows a reduced capacity to transform myelomonocytic cells at the nonpermissive temperature. Myelobiasts transformed by this mutant suffer a substantial decrease in the amount of the transforming protein p45v-myb when shifted from the permissive to the nonpermissive temperature. We presume that the 5-to 10-fold decrease in the amount of p45vmyb causes the loss of the transformed phenotype. The decrease is due to a reduction in the level of v-myb mRNA. Mutant GA907/7 thus provides genetic evidence that p45vmYb is the transforming protein of avian myeloblastosis virus and apparently represents an unusual defect in the production or stability of mRNA.Oncogenic transformation of myelomonocytic cells by avian myeloblastosis virus (AMV) is thought to be mediated by the expression of a viral oncogene termed v-myb (2, 5). The nucleotide sequence of v-myb represents a partial copy of a cellular gene, c-myb, and has been transduced into the viral genome from a chicken host (3,4,6,11,12). The viral gene encodes a 45,000-Mr protein, p45v-m b (1, 7), which in turn is a truncated version of the 75,000-Mr protein (p75cn`yb) encoded by chicken c-myb (7). Both proteins are located in the cell nucleus (8), but the means by which p45v-,n,b elicits oncogenic transformation and the function of its cellular counterpart, p75c-l,'b, remain obscure.Analysis of transformation-defective, temperaturesensitive mutants of oncogenic retroviruses has yielded valuable information about possible mechanisms of cell transformation. We have recently described a temperaturesensitive mutant of AMV (strain GA907/7, designated tsAMV hereafter), which shows a markedly reduced capacity to transform myeloid cells at the nonpermissive temperature (10). The results presented in this communication extend the characterization of this mutant to a biochemical analysis of viral mRNA and p45vni1b in cells transformed by tsAMV.The experiments to be described were all performed with a myeloblast cell line (GA950/2) that was derived by transforming chicken yolk sac macrophages with tsAMV (10). Cells of this line were cultivated in BT88 medium (9) supplemented with 10% tryptose phosphate broth, 5% calf serum, 5% chicken serum, and 1.6 ,ug of folic acid per ml. At the permissive temperature of 35.5°C, these cells proliferate and exhibit a blastlike morphology; when shifted to the nonpermissive temperature of 41°C, they cease to proliferate and attain a macrophagelike morphology within 2 to 5 days. * We first analyzed the level of p45v-,n,b at the permissive and nonpermissive temperatures by labeling with [35S]methionine. Myeloblasts transformed by tsAMV were incubated for 30 min in growth medium lacking methionine at either 35.5°C or 41°C (Fig. 1A). [35S]methionine (>600 Ci/mmol; Amersham Corp.) was then added to a final concentration of 500 ,uCi/ml, and the incubation was continued for 1 h at both temperatures. Cell lysates were prepared as described previously (7). Proteins of inter...