Differential template recognition by the Caulobacter crescentus and the escherichia coli RNA polymerases

Transcription of Escherichia coli and Caulobacter crescentus phage DNAs by their respective host RNA polymerase was examined to determine their ability to recognize specific transcription signals on the heterologous template. Analysis of coliphage T7 in vitro transcripts showed that, like the E. coli enzyme, the C. crescentus RNA polymerase initiated transcription from the three major T7 early promoters and recognized the terminator at the end of the early region. On the other hand, several differences were found between the C. crescentus and E. coli RNA polymerases with respect to their interaction with Caulobacter phage phiCdl DNA. The rates of open complex formation and RNA elongation were slower when phiCdl DNA was transcribed by the E. coli RNA polymerase. In addition, transcription of phiCdl DNA by the E. coli enzyme produced a subset of transcripts not synthesized by the C. crescentus enzyme. The production of these different transcripts by the E. coli enzyme was dependent on salt concentration and, in at least one case, appeared to be the result of differential termination. Although both enzymes protected the same sites on phiCdl DNA from cleavage with HincII, the E. coli enzyme was unable to form stable complexes with some phiCdl restriction fragments that formed stable complexes with the C. crescentus RNA polymerase. These results indicate that although the C. crescentus RNA polymerase can accurately recognize transcription signals on a heterologous phage template, the E. coli enzyme exhibits altered specificity with a heterologous phage template of higher G + C content.