Co-ordination of the expression of the protein tyrosine kinase p56(lck) with the pre-T cell receptor during thymocyte development

The protein tyrosine kinase p56(lck) is essential for the regulation of ear ly thymocyte development by transducing pre-T cell receptor (pre-TCR) media ted-signals. Here we have investigated the developmental timing of expressi on of p56(lck) in the thymus. We show that p56(lck) transcripts are detecta ble in each CD4(-)CD8(-) double-negative (DN) thymocyte subset defined by C D25 and CD44. The steady-state level of p56(lck) mRNA with these four DN su bsets is similar, varying by less than 2-fold. However, p56(lck) protein le vels are undetectable in DN subsets 1 and 2, but are at least 20-fold up-re gulated between DN subsets 2 and 3. Analysis of independent transgenic mous e lines in which the proximal p56(lck) promoter was used to drive expressio n of a green fluorescent protein (GFP) reporter gene revealed a lack of cor relation between GFP mRNA expression and transgene copy number and variable expression of GFP protein in different lines. Taken together these results show that the expression of the p56(lck) gene is developmentally regulated at the post-transcriptional level at the precise developmental stage at wh ich its upstream receptor complex, the pre-TCR, is expressed. These results also have implications for the interpretation of the phenotype of transgen ic mice which utilize the p56(lck) proximal promoter.