(fusion hybrid/globin expression/HEL cells/globin gene activation)

Somatic cell hybridization of mouse erythro- leukemia (MEL) cells and HEL cells, a human erythroleuke- mia line that produces fetal (y) but fails to express adult (,P) globin, was used to test whether the expression of the two hu- man globin genes is regulated cis or trans. An experimental approach using anti-human globin monoclonal antibodies for detection, efficient cloning, and monitoring of hybrids of in- terest was employed. Further characterization of hybrids used isoelectric focusing for detection of human globins and S1 nu- clease mapping. In contrast to the parental HEL line, all chro- mosome 11-retaining HEL-MEL hybrids expressed human f- globin, suggesting that the HEL P-globin genes (i) are tran- scriptionally competent, (ii) become activated in response to a positive trans-acting element within the MEL environment, and (iii) fail to express into the HEL environment because of either the absence of a positive trans-acting element or the presence of a trans-acting inhibitor of fl-globin gene expres- sion. In addition to P-globin, the primary HEL-MEL hybrids co-expressed y-globin; however, y-globin expression segregat- ed by subcloning so that secondary and tertiary clones either expressed only j3-globin or co-expressed y- and j-globin. The results of subcloning can be explained by assuming that y-glo- bin gene expression is controlled by a HEL cell-derived trans- acting element encoded by a gene not syntenic to chromosome 11 or by postulating that the HEL y-globin genes become ran- domly modified during the continuous proliferation of hy- brids.