Genetic organization of loci encoding proteins of NHEJ apparatus. The genome of S. meliloti Rm1021 consists of three replicons: one chromosome, two megaplasmids (pSymA and pSymB) (Galibert et al., 2001). A blast database search detected four ORFs encoding close homologues of bacterial Ku in megaplasmids: SMa0426 (sku1) in pSymA; SMb20686, SMb21406 and SMb21407 (sku2, sku3 and sku4 respectively) in pSymB. Six ORFs encoding LigD homologues were also identified: SMa0414, SMa0417 and SMa0424 (ligD1L, ligD1P and ligC) in pSymA; SMb20685 and SMb21044 (ligD2 and ligD3) in pSymB; and SMC03959 (ligD4) in the chromosome. Genes are represented by arrows showing the direction of transcription and coloured depending on the predicted function of their encoding proteins: purple, bacterial Ku protein; green, DNA polymerase domain; yellow, ATP-dependent DNA ligase domain; blue, DNA exonuclease domain. The regions deleted in sku deletion mutants are marked with red lines. Regions subcloned in pMP220 are marked with blue lines. Arrowheads represent locations where pJH104 derivatives were inserted.