SummaryThe recently discovered prokaryotic CRISPR/Cas defence system provides immunity against viral infections and plasmid conjugation. It has been demonstrated that in Escherichia coli transcription of the Cascade genes (casABCDE) and to some extent the CRISPR array is repressed by heat-stable nucleoidstructuring (H-NS) protein, a global transcriptional repressor. Here we elaborate on the control of the E. coli CRISPR/Cas system, and study the effect on CRISPR-based anti-viral immunity. Transformation of wild-type E. coli K12 with CRISPR spacers that are complementary to phage Lambda does not lead to detectable protection against Lambda infection. However, when an H-NS mutant of E. coli K12 is transformed with the same anti-Lambda CRISPR, this does result in reduced sensitivity to phage infection. In addition, it is demonstrated that LeuO, a LysR-type transcription factor, binds to two sites flanking the casA promoter and the H-NS nucleation site, resulting in derepression of casABCDE12 transcription. Overexpression of LeuO in E. coli K12 containing an antiLambda CRISPR leads to an enhanced protection against phage infection. This study demonstrates that in E. coli H-NS and LeuO are antagonistic regulators of CRISPR-based immunity.

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Ultraviolet B (UVB) radiation induces mutagenic DNA photoproducts, in particular cyclobutane pyrimidine dimers (CPDs), in epidermal keratinocytes (KC). To prevent skin carcinogenesis, these DNA photoproducts must be removed by nucleotide excision repair (NER) or apoptosis. Here we report that the UVB-sensitive transcription factor aryl hydrocarbon receptor (AHR) attenuates the clearance of UVB-induced CPDs in human HaCaT KC and skin from SKH-1 hairless mice. Subsequent RNA interference and inhibitor studies in KC revealed that AHR specifically suppresses global genome but not transcription-coupled NER. In further experiments, we found that the accelerated repair of CPDs in AHR-compromised KC depended on a modulation of the p27 tumor suppressor protein. Accordingly, p27 protein levels were increased in AHR-silenced KC and skin biopsies from AHR−/− mice, and critical for the improvement of NER. Besides increasing NER activity, AHR inhibition was accompanied by an enhanced occurrence of DNA double-strand breaks triggering KC apoptosis at later time points after irradiation. The UVB-activated AHR thus acts as a negative regulator of both early defense systems against carcinogenesis, NER and apoptosis, implying that it exhibits tumorigenic functions in UVB-exposed skin. In fact, AHR−/− mice developed 50% less UVB-induced cutaneous squamous cell carcinomas in a chronic photocarcinogenesis study than their AHR+/+ littermates. Taken together, our data reveal that AHR influences DNA damage-dependent responses in UVB-irradiated KC and critically contributes to skin photocarcinogenesis in mice.

Mescher et al. uncover a novel tissue-borne tumor suppression mechanism, engaging polarity proteins in the epithelial microenvironment that prevent malignant outgrowth of neighboring cell types through control of heterologous cell–cell contacts. Moreover, their data support an emerging role of P-cadherin, which is frequently amplified in human carcinoma, as a protumorigenic and proinvasive adhesion molecule, thus placing it as a promising druggable target to disrupt tumor–microenvironment interactions for anticancer therapy.