Protein kinase C regulation of terminal deoxynucleotidyl transferase

Terminal deoxynucleotidyl transferase (TdT) is a nuclear enzyme found in early lymphocytes which is thought to increase junctional diversity of TCR and Ig genes by the addition of N regions. TdT is normally found only in immature lymphoid cells and is turned off in both mature B and T cells. To investigate the regulation of TdT gene expression, pre-B and pre-T cells were treated with PMA or three of its analogs and its effects on steady-state TdT mRNA levels determined. Rapid and reversible decline in steady-state TdT mRNA levels was observed within 6 h with PMA. This rapid decline can be blocked by pretreatment of the cells with a relatively selective protein kinase C inhibitor implicating the role of protein kinase C activation in the decline of TdT mRNA. Nuclear run-off studies demonstrate that TdT transcription is rapidly down-regulated within 45 min after PMA treatment, indicating that this regulation occurs mainly at the level of transcription. Furthermore, the TdT mRNA decline is blocked in the presence of cycloheximide, showing that new protein synthesis is required for inactivation of the gene.