Every bloody protocol suggests adding in DTT when doing in vitro RNA transcription. Why? The rationale seems to be that the cytoplasm traditionally has a reducing environment but as the only protein we care about is the T7 polymerase, why is this necessary.
A quick search on T7 cysteines gave some clues:
Bacteriophage T7-induced DNA polymerase is composed of a 1: 1 complex of phage-induced gene 5 protein and Escherichia coli thioredoxin. Preparation of active subunits in the absence of sulfhydryl reagents indicates the reduced form of thioredoxin is sufficient for formation of the active holoenzyme. The oxidized form of thioredoxin, thioredoxin modified at one active site sulfhydryl by iodoacetate or methyl iodide, or thioredoxin modified at both active site sulfhydryls by N-ethylmaleimide, are all inactive, being defective in complex formation with gene 5 protein.
There's a more recent paper (Aguirre et al, Inorganic Chemistry 48:4425 (2009)) that mentions the "the enzyme critical sulfhydryl cysteine groups", but unfortunately I only have access to the abstract.
Update: It seems to be an old finding, rather than a rationale concerning the cytoplasmic redox state. According to Chamberlin and Ring, JBC 248:2235 (1973),
General Requirements-The general requirements for T7 RNA synthesis directed by T7 DNA polymerase are shown in Table I. As expected for a template directed polymerase, RNA synthesis shows an absolute requirement for DNA, the 4 ribonucleoside triphosphates and Mg++.
(no surprises there ;)
The activity of the enzyme is reduced significantly if a sulfhydryl reducing agent such as b-mercapto-ethanol is omitted from the reaction. The addition of 10^-5 M p-hydroxymercuribenzoate to the assay system in the absence of b-mercaptoethanol abolished all activity, indicating that the enzyme contains a sulfhydryl group necessary for activity.
However, if you see the table I, the remaining activity after removing bme is still 74%
There seems to be 7 exposed cysteines (Mukherjee et al, Cell 110:81 (2002)), but I could not find any paper discussing their roles.
I always assumed it was because DTT is useful in inactivating ribonucleases (by reducing their disulfides) which are notoriously stable and pervasive. It would be pretty unfortunate to get your RNA synthesized only to have it immediately be destroyed by a contaminating RNase.