{'fraction_non_alphanumeric': 0.026380873866446827, 'fraction_numerical': 0.016488046166529265, 'mean_word_length': 5.100502512562814, 'pattern_counts': {'":': 0, '<': 1, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Characterization of Cdk9 T‐loop phosphorylation in resting and activated CD4+ T lymphocytes

The cellular kinase complex P‐TEFb is composed of Cdk9 and cyclin T, and it is required for expression of most protein‐coding genes by RNAP II. Cdk9 has been shown recently to be activated in cis by autophosphorylation of Thr186 in its T‐loop. Using a phosphospecific Cdk9 antibody, we examined the level of Cdk9 T‐loop phosphorylation in resting and activated CD4+ T lymphocytes. Cdk9 T‐loop phosphorylation was found to be low‐to‐undetectable in resting CD4+ T lymphocytes, and upon activation by distinct stimuli, there is a rapid (<1 h) increase in pCdk9 that does not require protein synthesis. The low level of Cdk9 T‐loop phosphorylation was not to be a result of the absence of an associated regulatory cyclin partner. These observations suggest that autophosphorylation of the Cdk9 T‐loop is repressed in resting CD4+ T lymphocytes. The low level of T‐loop phosphorylation in resting cells is also reflected in a low level of phosphorylation of Ser2 in the carboxyl terminal domain of RNAP II, suggesting that lack of Cdk9 T‐loop autophosphorylation may limit RNAP II elongation in quiescent CD4+ T lymphocytes.