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
Adler and Modrich, J Biol Chem 258:6956 (1983)
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.