Differential regulation of NF-kappaB by elongation factors is determined by core promoter type.

NF-kappaB transcription factors activate genes important for immune response, inflammation, and cell survival. P-TEFb and DSIF, which are positive and negative transcription elongation factors, respectively, both regulate NF-kappaB-induced transcription, but the mechanism underlying their recruitment to NF-kappaB target genes is unknown. We show here that upon induction of NF-kappaB, a subset of target genes is regulated differentially by either P-TEFb or DSIF. The regulation of these genes and their occupancy by these elongation factors are dependent on the NF-kappaB enhancer and the core promoter type. Converting a TATA-less promoter to a TATA promoter switches the regulation of NF-kappaB from DSIF to P-TEFb. Accumulation or displacement of DSIF and P-TEFb is dictated by the formation of distinct initiation complexes (TFIID dependent or independent) on the two types of core promoter. The underlying mechanism for the dissociation of DSIF from TATA promoters upon NF-kappaB activation involves the phosphorylation of RNA polymerase II by P-TEFb. The results highlight a regulatory link between the initiation and the elongation phases of the transcription reaction and broaden our comprehension of the NF-kappaB pathway.

p53 and p21 regulate error-prone DNA repair to yield a lower mutation load.

Regulation of mutation rates is critical for maintaining genome stability and controlling cancer risk. A special challenge to this regulation is the presence of multiple mutagenic DNA polymerases in mammals. These polymerases function in translesion DNA synthesis (TLS), an error-prone DNA repair process that involves DNA synthesis across DNA lesions. We found that in mammalian cells TLS is controlled by the tumor suppressor p53, and by the cell cycle inhibitor p21 via its PCNA-interacting domain, to maintain a low mutagenic load at the price of reduced repair efficiency. This regulation may be mediated by binding of p21 to PCNA and via DNA damage-induced ubiquitination of PCNA, which is stimulated by p53 and p21. Loss of this regulation by inactivation of p53 or p21 causes an out of control lesion-bypass activity, which increases the mutational load and might therefore play a role in pathogenic processes caused by genetic instability.

Conditional inactivation of Pax6 in the pancreas causes early onset of diabetes.

Pax6 transcription factor is required for islet cell number, morphology, and hormone gene expression. The perinatal lethality of Pax6 null mutants has restricted investigation of the role of Pax6 in normal endocrine cell function. Therefore, we devised the conditional inactivation of Pax6 using the Pdx1 and Pax6 regulatory domains to activate Cre in cells of either the entire pancreatic bud or only in endocrine cell lineages, respectively. Mutant pups died few days after birth, suffering from an overt diabetic phenotype that includes hyperglycemia, hypoinsulinemia, weight loss, and ketosis, indicating an essential role for Pax6 in beta cell function. Glucose-transporter type-2 expression was downregulated, but expression of several transcription factors essential for endocrine development was maintained. Our findings support a role for Pax6 activity in maintaining normal beta cell function after birth, but not for beta cell neogenesis during late embryonic development and early postnatal stages.