Experiments dealing with the regulation of psbA-I gene expression in Synechocystis 6803 have produced results that indicate interchromosomal recombination events. A kanamycin resistance reporter gene (KmR) was inserted within the psbA-I structural gene, and a chloramphenicol resistance gene (CmR) coupled to an origin of replication for E. coli was added downstream to the KmR reporter, thus allowing rescue of this entire DNA region. Transformation of wild-type (WT) Synechocystis 6803 cells with this plasmid produced only very few KmR transformants. However, random deletions in the region between the putative psbA-I promoter and the KmR reporter led to a marked increase in the number of KmR transformants obtained. Plasmids, rescued from such transformants, were used for secondary transformations. The rescued plasmids were consistently effective in rendering KmR. Moreover, the transformants were found to contain two distinct chromosomal configurations; one containing the deletion as opposed to the other which remained intact. Molecular analysis of the various KmR transformants suggests a series of interchromosomal recombinations, a model for which is provided.
- homologous recombination