BACKGROUND: Extracellular protein production by Gram-positive bacteria, such as Streptomyces, may be complementary to current established protein production processes. The performance of a Streptomyces lividans mutant strain, deficient in the major signal peptidase (SipY) is investigated for the production of proteins secreted via the secondary Tat pathway. RESULTS: The SipY deficient strain has shown advantages over the wild type strain, in terms of extracellular productivity, specific activity and rheological behaviour. Two operational modes, batch and fed-batch, have been studied using mannitol as carbon source. The results showed that two successive mannitol additions in fed-batch mode led to improved secretory protein production using Streptomyces agarase as a model protein. This production process was also explored for the Tat secretory protein S. lividans lactase. The predicted sequence for the pre-lactase coding sequence has been cloned into the mutant strain under the control of the agarase promoter. Batch and fed-batch lactase production, using either mannitol or glucose as carbon source, have been developed and quantified. CONCLUSIONS: The usefulness of a Streptomyces lividans SipY deficient strain as protein producer has been demonstrated. A proposed operating mode with substrate additions has been employed for the optimisation of Tat proteins production, although some adjustments might be necessary depending on the secretory protein. (C) 2016 Society of Chemical Industry