Expression of the plant sulphite reductase in cell suspension cultures from Catharanthus roseus L.

Sulphur-heterotrophic growth exhibited a dual response to the expression of sulphate-assimilating enzymes. The level of ATP-sulphurylase (EC 2.7.7.4) appeared "repressed" while sulphite reductase (EC 1.8.7.1) and O-acetyl-L-serine sulphhydrylase (EC 4.2.99.8) were "derepressed" and coordinated in their occurrence. The capability of the cells to reduce adenylylphosphosulphate or 3'-phospho adenylylphosphosulphate to cysteine coincided with the activity of sulphite reductase. The expression of these reducing steps lacked correlation with the regulation of ATP-sulphurylase.