Bacteria synthesize a number of different sigma factors which allow the co-ordinate expression of groups of genes owing to the ability of sigma to confer promoter-specific transcription initiation on RNA polymerase. In nearly all cases these sigmas belong to a single family of proteins which appear to be related structurally and functionally to the major Escherichia coli sigma factor, sigma 70. A clear exception is the sigma factor sigma 54 (sigma N), encoded by rpoN, which represents a second family of sigmas that is widely distributed in prokaryotes. Studies of sigma 54 (sigma N) have demonstrated that this sigma is quite distinct both structurally and functionally from the sigma 70 family and the mode of transcription initiation which it mediates may have more in common with that found in eukaryotes than that which occurs with sigma 70 and its relatives.