Bacillus popilliae cry18Aa operon is transcribed by sigmaE and sigmaK forms of RNA polymerase from a single initiation site

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Abstract
Bacillus popilliae is an obligate pathogen for larvae of the insect family Scarabaeidae (Coleoptera). It forms parasporal crystals upon sporulation. The gene cry18Aa coding for the parasporal crystal protein and an upstream open reading frame, orf1, were previously isolated from B.popilliae. Here we report an analysis of cry18Aa transcription in Bacillus thuringiensis. The only transcriptional start site of cry18Aa was found 29 bp upstream of the open reading frame orf1, suggesting that orf1 and cry18Aa are transcribed as an operon. lacZ fusion to the cry18Aa promoter was used to follow the time-course of cry18Aa transcription in wild type B.thuringiensis and in various B.thuringiensis sporulation-deficient mutants (spo0A, sigE or sigK). In wild type B.thuringiensis, the cry18Aa promoter was activated 2 h after the end of exponential growth and the expression lasted to the late sporulation phase. The results of promoter activity in Spo+ or Spo− backgrounds together with the results of primer extension experiments suggest that the transcription from this promoter can be driven by both σE and σK types of RNA polymerase at a single start site. The promoter region of cry18Aa operon fits the consensus sequences of both σE and σK dependent promoters of Bacillus.