FIGURE 4.

Igα expression in BAC transformants. A and B, Northern blots of RNA from an Igα producing plasmacytoma (J558), the γ2a-producing parental line (9921), and the 9921 BAC transformants (1.1, 6.4, etc) (A). Duplicate gels were transferred to nylon and sequentially hybridized with probes for Cα and GAPDH or for Cγ2a and GAPDH, respectively. Representative blots are shown (experiment repeated three times). Estimated BAC copy number shown below each lane (the single-copy J558 endogenous gene is represented as 1). A, 9921 transformants carrying B1-8α. B, 9921 transformants carrying Blxhs4. (Note that specific activities of the GAPDH and Igα probes were not held constant from experiment to experiment, so the relative signals vary among experiments; e.g., compare A and B. This has no adverse effect on normalizing samples to GAPDH within an individual experiment.) C, Plot of Igα mRNA levels vs transgene copy number for nine Blxhs4 transformants. Igα mRNA levels were normalized to GAPDH and the level for each clone was divided by that for J558 in each experiment (to normalize among experiments). The SD for each clone (from three determinations) is shown. The best-fit line of linear regression is shown (linear regression by least squares criterion (Data Desk); p = 0.009, R² = 0.18). D, DNase I hypersensitivity sites 3b and 4 in the Blxhs4 transgene. Southern blot of BglII/SalI-digested DNA hybridized with the β-globin “tag” as the probe and treated with increasing levels of DNase I. Representative data from one of the Blxhs4 transformants (137.1) is shown. Below the autoradiograph is a diagram of the region analyzed. The approximate sizes of the DNase I fragments expected for hs3b and hs4 are indicated (~3.2 and ~1.2 kb, respectively).