And Philip W

Messenger R N A that code for the /x chain of secreted (s) and cell surface (m) immunoglobul in M (IgM) differ in size ~ m R N A are 2.4 kilobases [kb]; ~m m R N A are 2.7 kb) and in sequence at their 3' termini (1-4). These sequences predict two different carboxy terminal amino acid sequences and account for peptide (5-7) as well as hydrophobici ty (8, 9) differences that have been shown for the two molecules. The two R N A are, however, transcribed from a single Ckt gene (10) by alternative recognition of different polyadenylat ion sites. The mechanism for the regulation of the level of the two m R N A within different B cell populat ions is not known, a l though myeloma and hybr idoma cells that are actively secreting Ig (1, 2) as well as fetal liver hybridomas (11) that synthesize only cytoplasmic/~ chains contain more of the 2.4 kb m R N A , whereas l y m p h o m a cells (1, 2) and Abelson virus-transformed pre-B cell lines (12) contain approximately equal amounts of both RNA. In an a t tempt to unders tand the regulation of the transcription and translation of these two m R N A as a cell is st imulated from a quiescent state to that of active secretion, we examined changes in the amount of both/.t-specific m R N A and polypept ide in a B cell tumor line, BCL1 (13), before and after induction to Ig synthesis by a mitogen. We chose the in vivo line of BCL1 for these studies because these cells are closely analogous to the relatively immature B lymphocyte in that they express a high I g M / IgD ratio and do not secrete IgM (14, 15). They can, however, be act ivated by mitogens such as lipopolysaccharides (LPS) 1 and will secrete 19s IgM that has the same idiotype as the cell surface IgM (15, 16). The in vivo line of BCL1, therefore, differs from the line that has been adapted to grow in vitro (17) in that the latter secretes IgM at a low level without addit ional st imulation and has been shown to contain/Xm m R N A and/x~ m R N A in approximately equal amounts (1). In contrast, in the present studies we show that the/~ chain m R N A made by the in vivo cell line is predominant ly 2.7 kb in size, suggesting that the cells are not in an activated state in regard to Ig secretion. We are, therefore, able to analyze regulation of/~ m R N A synthesis in this cell line. Our studies document that, upon st imulation to secretion