PRINCIPAL SIGMA-SUBUNIT OF THE CAULOBACTER-CRESCENTUS RNA-POLYMERASE

We have identified the gene encoding the Caulobacter crescentus principal sigma subunit, RpoD. The rpoD gene codes for a polypeptide of 653 amino acids with a predicted molecular mass of 72,623 Da (sigma(73)), The C. crescentus sigma subunit has extensive amino acid sequence homology with the principal sigma factors of a number of divergent procaryotes, In particular, the segments designated region 2 that are involved in core polymerase binding and promoter recognition were identical among these bacteria despite the fact that the -10 region recognized by the C. crescentus sigma(73) differs significantly from that of the other bacteria. Thus, it appears that additional sigma factor regions must be involved in -10 region recognition, This conclusion was strengthened by a heterologous complementation assay in which C. crescentus sigma(73) was capable of complementing the Escherichia coli rpoD285 temperature-sensitive mutant. Furthermore, C. crescentus sigma(73) conferred new specificity on the E. coli RNA polymerase, allowing the expression of C. crescentus promoters in E. coli, Thus, the C. crescentus sigma(73) appears to have a broader specificity than does the sigma(70) of the enteric bacteria.