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Background  Crocin is a carotenoid-derived natural product found in the stigma of  Crocus  spp., which has great potential in medicine, food and cosmetics. In recent years, microbial production of crocin has drawn increasing attention, but there were no reports of successful implementation.  Escherichia coli  has been engineered to produce various carotenoids, including lycopene, β-carotene and astaxanthin. Therefore, we intended to construct  E. coli  cell factories for crocin biosynthesis.    Results  In this study, a heterologous crocetin and crocin synthesis pathway was first constructed in  E. coli . Firstly, the three different zeaxanthin-cleaving dioxygenases  Cs ZCD,  Cs CCD2 from  Crocus sativus , and  Ca CCD2 from  Crocus ancyrensis , as well as the glycosyltransferases UGT94E5 and UGT75L6 from  Gardenia jasminoides , were introduced into zeaxanthin-producing  E. coli  cells. The results showed that  Cs CCD2 catalyzed the synthesis of crocetin dialdehyde. Next, the aldehyde dehydrogenases ALD3, ALD6 and ALD9 from  Crocus sativus  and ALD8 from  Neurospora crassa  were tested for crocetin dialdehyde oxidation, and we were able to produce 4.42 mg/L crocetin using strain YL4(p Cs CCD2-UGT94E5-UGT75L6,pTrc-ALD8). Glycosyltransferases from diverse sources were screened by in vitro enzyme activity assays. The results showed that crocin and its various derivatives could be obtained using the glycosyltransferases YjiC, YdhE and YojK from  Bacillus subtilis , and the corresponding genes were introduced into the previously constructed crocetin-producing strain. Finally, crocin-5 was detected among the fermentation products of strain YL4(p Cs CCD2-UGT94E5-UGT75L6,pTrc-ALD8,pET28a-YjiC-YdhE-YojK) using HPLC and LC–ESI–MS.    Conclusions  A heterologous crocin synthesis pathway was constructed in vitro, using glycosyltransferases from the  Bacillus subtilis  instead of the original plant glycosyltransferases, and a crocetin and crocin-5 producing  E. coli  cell factory was obtained. This research provides a foundation for the large-scale production of crocetin and crocin in  E. coli  cell factories.    Electronic supplementary material  The online version of this article (10.1186/s12934-019-1166-1) contains supplementary material, which is available to authorized users.

Construction of Escherichia coli cell factories for crocin biosynthesis