Polycistronic transcription of fused cassettes and identification of translation initiation signals in an unusual gene cassette array from Pseudomonas aeruginosa

The gene cassettes found in class 1 integrons are generally promoterless units composed by an open reading frame (ORF), a short 5' untranslated region (UTR) and a 3' recombination site ( attC). Fused gene cassettes are generated by partial or total loss of the attC from the first cassette in an array, creating, in some cases, a fusion with the ORF from the next cassette. These structures are rare and little is known about their mechanisms of mobilization and expression. The aim of this study was to evaluate the dynamic of mobilization and transcription of the gcu14-bla GES-1 /aacA4 gene cassette array, which harbours a fused gene cassette represented by bla GES-1 /aacA4. The cassette array was analyzed by Northern blot and real-time reverse transcription-polymerase chain reaction (RT-PCR) in order to assess the transcription mechanism of bla GES-1 /aacA4 fused cassette. Also, inverse polymerase chain reactions (PCR) were performed to detect the free circular forms of gcu14, bla GES-1 and aacA4. The Northern blot and real time RT-PCR revealed a polycistronic transcription, in which the fused cassette bla GES-1 /aacA4 is transcribed as a unique gene, while gcu14 (with a canonical attC recombination site) has a monocistronic transcription. The gcu14 cassette, closer to the weak configuration of cassette promoter (PcW), had a higher transcription level than bla GES-1/ aacA4, indicating that the cassette position affects the transcript amounts. The presence of ORF-11 at attI1, immediately preceding gcu14, and of a Shine-Dalgarno sequence upstream bla GES-1/ aacA4 composes a scenario for the occurrence of array translation. Inverse PCR generated amplicons corresponding to gcu14, gcu14-aacA4 and gcu14-bla GES-1/ aacA4 free circular forms, but not to bla GES-1 and aacA4 alone, indicating that the GES-1 truncated attC is not substrate of integrase activity and that these genes are mobilized together as a unique cassette. This study was original in showing the transcription of fused cassettes and in correlating cassette position with transcription.