Identification of promoter activity in gene-less cassettes from Vibrionaceae superintegrons
Integrons are genetic platforms that acquire new genes encoded in integron cassettes (ICs), building arrays of adaptive functions for bacteria. ICs generally encode promoterless genes, whose expression relies on the PC promoter within the integron platform. Cassette arrays are assumed to be operon-like structures in which expression is dependent on the distance to the Pc. This is especially relevant in large sedentary chromosomal integrons (SCIs,) like the ones in Vibrio species. We have identified 29 gene-less cassettes in 4 Vibrio SCIs, and explored whether their function could be related to regulating the transcription of adjacent ICs. We show that most gene-less cassettes have promoter activity on the sense strand, enhancing the expression of downstream cassettes. Accordingly, we found that most of the superintegron in Vibrio cholerae is not silent. These promoter cassettes can trigger the expression of a silent dfrB9 resistance cassette downstream, increasing trimethoprim resistance >512-fold in V. cholerae and Escherichia coli. Additionally, one cassette had an antisense promoter capable of reducing trimethoprim resistance through transcriptional interference. Our findings highlight the regulatory role of gene-less cassettes in the expression of adjacent cassettes, emphasizing their significance in large SCIs and their clinical importance if captured by mobile integrons. Overall design: Gene expression analysis of RNA-seq data for Vibrio cholerae N16961 strain at exponential (OD600 0.8) and stationary (OD600 2.8) growth phase at 37°C in LB medium. Three biological replicates per condition are included.