We examined the turnover of c-myc RNA in the human promyelocytic cell line HL-60. In whole-cell RNA from rapidly growing cells we observed two major size classes of c-myc RNA, 2.4 and 2.2 kilobases (kb). When HL-60 cells were treated with actinomycin D for 30 min to inhibit transcription, the 2.4-kb c-myc RNA population was rapidly degraded, while the 2.2-kb c-myc RNA was degraded much more slowly. Si nuclease transcript mapping and promoter-specific probes were utilized to show that both the stable 2.2-kb and the labile 2.4-kb c-myc RNA populations have 5' ends at the second promoter site (P2) and 3' ends at the second poly(A) addition site. To examine further possible structural differences between these two RNA populations, we fractionated RNA on an oligo(dT)-cellulose column to separate RNAs that contained long poly(A) tails from those which did not. We found that the labile 2.4-kb c-myc RNA population bound to oligo(dT)-cellulose, while the more stable 2.2-kb c-myc RNA population did not. Preliminary estimates of their half-lives (tl/2) showed that the poly(A)+ c-myc RNA had a tl/2 of 12 min, while the c-myc RNA that did not bind to oligo(dT)-cellulose had a t1/2 of >1 h. Several other cell types contain both poly(A)+ and nonpoly(A)+ c-myc RNAs including HeLa cells, normal human bone marrow cells, and normal mouse fetal liver cells. In agreement with the results in HL-60 cell, HeLa cell poly(A)+ c-myc RNA was more labile than c-myc RNA that lacked poly(A). The stable, nonpoly(A)+ c-myc RNA population may be important in the posttranscriptional regulation of c-myc expression.The cellular proto-oncogene c-myc is expressed in many cell types. c-myc expression is highest in proliferating cells, and it is turned off in differentiating cells (13,19,22,30,31). The c-myc protein is localized to the cell nucleus where it is believed to participate in the regulation of cellular growth (15). c-myc is the cellular homolog of several avian retroviruses, including the prototypic MC29 virus. The c-myc gene has been implicated in the genesis of several hematopoietic neoplasias. It is translocated to the immunoglobulin heavychain locus in Burkitt lymphomas and in mouse plasmacytomas (27)(28)(29). In human myeloid and lymphoid leukemias, a subset of patients show highly elevated c-myc RNA levels (24).c-myc gene regulation in fibroblasts and tumor cells in culture has been shown to be, in part, posttranscriptional. A human c-myc mRNA half-life of 20 to 30 min has been reported for several cell lines in culture, and this half-life can be modulated by interferons (9,10,14). The authors of these studies, however, utilized poly(A)+ RNA prepared from whole cells as their source of c-myc RNA and not total cellular RNA. We investigated the turnover of c-myc mRNA using total cellular RNA from the human promyelocytic leukemia cell line HL-60. We which does not bind to oligo(dT)-cellulose. Taken together these data suggest that poly(A) is a structural feature of c-myc mRNA that is used to regulate its steady-state level. MATERIALS AN...