Sigma factor

A sigma factor (σ factor) is a protein needed only for initiation of RNA synthesis. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase to gene promoters. The specific sigma factor used to initiate transcription of a given gene will vary, depending on the gene and on the environmental signals needed to initiate transcription of that gene.

Every molecule of RNA polymerase holoenzyme contains exactly one sigma factor subunit, which in the model bacterium Escherichia coli is one of those listed below. The number of sigma factors varies between bacterial species.E. coli has seven sigma factors. Sigma factors are distinguished by their characteristic molecular weights. For example, σ⁷⁰ refers to the sigma factor with a molecular weight of 70 kDa.

RNA polymerase holoenzyme complex consisting of core RNA polymerase and a sigma factor executes transcription of a DNA template strand. Once initiation of RNA transcription is complete, the sigma factor can leave the complex.

Sigma

Sigma (upper-case Σ, lower-case σ, lower-case in word-final position ς; Greek σίγμα) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. When used at the end of a word, when the word is not all upper-case, the final form (ς) is used, e.g. Ὀδυσσεύς (Odysseus); note the two sigmas in the center of the name, and the word-final sigma at the end.

History

The shape and alphabetic position of Sigma is derived from Phoenician shin 𐤔 .

Etymology

The name of sigma, according to one hypothesis, may continue that of Phoenician Samekh. According to a different theory, its original name may have been "San" (the name today associated with another, obsolete letter), while "sigma" was a Greek innovation that simply meant "hissing", based on a nominalization of a verb σίζω (sízō, from earlier *sig-jō, meaning 'I hiss').

Uppercase of esh

The uppercase form of sigma was re-borrowed into the Latin alphabet to serve as the uppercase of modern esh (lowercase: ʃ).