IN A CLASS OF ITS OWN - THE RNA-POLYMERASE SIGMA-FACTOR SIGMA(54) (SIGMA(N))

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, sigma70. A clear exception is the sigma factor sigma54 (sigma(N)), encoded by rpoN, which represents a second family of sigmas that is widely distributed in prokaryotes. Studies of sigma54 (sigma(N)) have demonstrated that this sigma is quite distinct both structurally and functionally from the sigma70 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 sigma70 and its relatives.