## 4 Discussion
Comparing the substrate specificity of GGPP synthases from _Microoccus lysodekitcus_ and pumpkin seedling, it is found that the enzyme in pumpkin seedling has the highest activity, while the lowest activity is in _Microoccus lysodekitcus_[49]. It is consistent with the analysis results that GGPP synthases are derived from different organisms with widely different properties. In the third stage, GGPP is transferred to lutein and zeaxanthin in carotenoid biosynthesis pathways (Figure 4). Six out of at least nine enzymes involved in the lutein and zeaxanthin biosynthesis are annotated into the genomes of two strains (Figure S5). CrtB catalyzes the conversion of GGPP to prephytoene diphosphate. Evidence for the presence of the _crtB_ and the _crtB_ operon was found in the purple photosynthetic bacterium _Rhodobacter spheroides_[50]. At the same time, ORF-A protein homologous to the amino acid sequence of CrtB was found in the _Thermus thermophilus_, heterologous expression showed that the strain containing ORF-A protein had three times the carotenoid production of the strain without containing ORF-A protein [51]. Further experiments showed that the CrtB enzyme in _Erwinia uredovora_ requires Mn\({}^{2+}\) and had an optimum pH of 8.2 to produce the only product phytoene. CrtB is a highly conserved enzyme that can be used to design new drugs or pesticides with specific targets [52]. Two desaturases are annotated in the genomic data of _Sphingomonas morindae_ sp. NBD5 and _Sphingopyxis_ sp. USTB-05. Crtla catalyzes the conversion of phytoene in the cis conformation to \(\zeta\)-carotene in two steps. Crtlb catalyzes the conversion of \(\zeta\)-carotene in the trans conformation to lycopene. The complete _crtl_ gene of _Erwinia_ was heterologously expressed in _E. coli_, and the purified enzyme could catalyze the conversion of cis-phytoene into translycopene as well as to bisdehydrolycopene [53]. The complete _crtl_ gene was overexpressed in _E. coli_, _Rhodobacter capsulatus_, and _Rhodobacter spheroides_, which catalyzed the desaturation of phytoene to produce neurosporene. It is an ATP-dependent enzyme [50; 54]. Further studies found that there were two different forms of Crtla and Crtlb in _Myxococcus xanthus_. Crtla catalyzes the dehydrogenation of caroten in the cis conformation, and Crtlb catalyzes the dehydrogenation of caroten in the trans conformation [55]. Two cyclases are found in the genomic data of _Sphingomonas morindae_ sp. NBD5 and _Sphingopyxis_ sp. USTB-05. Crtl-b is responsible for three step cyclization reaction. Unannotated enzymes catalyze the conversion of lycopene to \(\delta\)-carotene (Figure 4). CrtL-b catalyzes the cyclization of lycopene to form different types of carotenes. Earlier reports showed that CrtL catalyzed the conversion of acyclic lycopene to bicyclic \(\beta\)-carotene [56]. The predicted lycopene cyclases in rice are CrtL-b and CrtL-e proteins. They are only about 34% and 50% similar to CrtL from _Synechococcus_ sp. PCC 7942 [56]. Other types of lycopene cyclases are reported, lycopene cyclase genes from _Erwinia (crtl)_ or the plant capsicum (_lcy_) can catalyze monocyclic \(\beta\)-carotene produce bicyclic carotenoids 7,8-dihydro-\(\beta\)-carotene [57]. A fourth class of lycopene cyclases are found in photosynthetic bacteria, CruA and CruP exhibit lycopene cyclase activity [58]. Two separate Lycopene epsilon-cyclases (EC: 5.5.1.18) are identified for the first time in the marine cyanobacterium _Prochlorococcus marinus_ MED4. The enzyme crtL-e is responsible for the formation of cyclic carotenoids with \(\beta\)- or \(\varepsilon\)-terminal groups [59]. Although the cyclase crtL-e is not annotated in _Splingomonas morindae_ sp. NBD5 and _Splingopyxis_ sp. USTB-05, new genes distantly related to this cyclase is found based on the overall metabolic process. Three hydroxyles are found in the genomic data of _Splingomonas morindae_ sp. NBD5 and _Splingopyxis_ sp. USTB-05. CrtR-b and CrtZ are responsible for four steps hydroxylation.