Discovery of the extracytoplasmic function σ factors

Abstract

This special issue of Molecular Microbiology marks the 25^th anniversary of the discovery of the extracytoplasmic function (ECF) σ factors, proteins that subsequently emerged as the largest group of alternative σ factors and one of the three major pillars of signal transduction in bacteria, alongside one- and two-component systems. A single bacterial genome can encode > 100 ECF σ factors, and combined with their cognate anti-σ factors, they represent a modular design that primarily functions in transmembrane signal transduction. Here, we first describe the immediate events that led to the 1994 publication in the Proceeding of the National Academy of Sciences USA, and then set them in the broader context of key events in the history of σ biology research.

Fig. 1.

Fax sent by Mike Lonetto to Mark Buttner, dated 19^th May 1993, showing an alignment of the first 3 recognized ECF σ factors: Streptomyces σ^E (New28kd), Myxococcus xanthus CarQ (Mxcarqrs_2) and Pseudomonas aeruginosa AlgU (Psealgu_1). Mike’s handwriting can be seen on the fax, saying “Could this be a new sigma factor subfamily?”

Fig. 2.

The phylogenetic tree of the σ⁷⁰ family taken from Lonetto et al. (1994), incorporating 7 of the first 8 members of the newly identified ECF σ factor subfamily (Pseudomonas syringae HrpL was identified as an ECF σ factor in the paper but was not included in the phylogenetic analysis). The inclusion of the ECF σs essentially doubled the diversity of the σ⁷⁰ family.