THE HSD LOCI OF MYCOPLASMA-PULMONIS - ORGANIZATION, REARRANGEMENTS AND EXPRESSION OF GENES

Two paralogous, site-specific invertible loci, designated hsd1 and hsd 2 (host Specificity determinant), have been identified in the Mycoplas ma pulmonis genome, They encode putative type I restriction and modifi cation (R-M) systems with maximum sequence homology to the type IC fam ily, which includes EcoR124II and EcoDXXI, Each locus encodes an endon uclease subunit (HsdR), a methylase subunit (HsdM) and two DNA specifi city subunits (HsdS). The gene organization at each locus is such that hsdR and hsdM are flanked by two hsdS genes, Within each locus, one o f the hsdS genes, hsdR and hsdM, is encoded in tandem by the same DNA strand, while the second hsdS gene is encoded by the complementary str and but without overlap with the other three hsd genes, The hsdR and h sdM sequences of one locus are almost identical to their counterparts in the other, The four hsdS genes (two per locus) are highly homologou s at their 5' ends and also share sequence similarities in the 3' ends of their corresponding coding regions, Owing to the disposition of an d sequence similarities among the hsdS genes, they form inverted repea ts at each locus, Analysis by polymerase chain reaction (PCR) has show n that both loci behave as site-specific DNA invertible elements with multiple inversion sites, termed 'vipareetus', occurring within the hs dS genes. The inversions lead to a reassortment of hsdS sequences, gen erating an array of recombinant genes that probably encode S subunits possessing alternative DNA-binding specificities, Sequence information obtained from the analysis of hsd2 transcripts by 5' RACE (rapid ampl ification of cDNA ends) indicates that inversion induces the transcrip tion of alternative hsdS genes by the relocation of coding sequences d ownstream of a promoter and ribosome-binding site (RES) situated at on e end of each locus.