A mutant of Synechococcus PCC 7942 impaired in HCO3- uptake

An inactivation library was used to isolate high-CO2-requiring mutants of Synechococcus PCC 7942. One of them, mutant IL-7, is composed of elongated cells, some 5-15 times longer than the wild-type. IL-7 is impaired in the ability to accumulate inorganic carbon within the cells due to a lesion in HCO3- transport. Consequently, the apparent photosynthetic affinity for external inorganic carbon was about 50-100-fold lower than in the wild-type. Analysis of the genomic region modified in IL-7 demonstrated that the inactivating fragment was composed of two genomically unrelated fragments which were ligated together during the formation of the inactivation library. One of the fragments originated from a known genomic region, rbcLS, encoding ribulose 1,5-bisphosphate carboxylase/oxygenase and the other showed high homology to mutS encoding a DNA mismatch repair protein. We suggest that the primary lesion in IL-7 was in mutS and not in rbcLS, and that the phenotype of IL-7 resulted from secondary random mutations. We were unable to identify the spontaneous mutation(s) due to low transformability of IL-7. Our finding that two unrelated fragments ligated together points to possible mistakes in the identification of the function of putative genes with the aid of an inactivation library. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.