High-resolution physical map of the pSymb megaplasmid and comparison of the three replicons of Sinorhizobium meliloti strain 1021

A high-resolution physical map of the larger megaplasmid (pSymb) of Sinorhi zobium meliloti strain 1021 has been constructed by using BAC libraries and an original two-step PCR screening method. This method, previously used to map both the chromosome and the smaller megaplasmid (pSyma), allowed us to position over the genome a total of 842 markers with an average density of one marker every 8.3 kb. In addition, we used BLASTX and PRODOM analysis t o predict a function for a number of STSs, This work led to the discovery o f several interesting loci and to a comparison of the genetic information c arried by each replicon. The two main results emerging from this study are (i) a biased distribution of housekeeping genes, mainly detected on chromos ome, and (ii) the presence of an unexpected number of transporters, mainly belonging to the ABC superfamily. These are broadly distributed across the whole genome, but particularly found on pSymb.