The mrr gene of Escherichia coli K-12 is involved in the acceptance of foreign DNA which is modified. The introduction of plasmids carrying the HincII, HpaI, and TaqI R and M genes is severely restricted in E. coli strains that are Mrr+. A 2-kb EcoRI fragment from the plasmid pBg3 (B. Sain and N. E. Murray, Mol. Gen. Genet. 180:35-46, 1980) DNA (39,41,44,45). These two systems were initially called rglA and rglB because they restricted glucoseless, 5-hydroxylmethyl cytosine-containing DNA, present in many T-even phage (42). One system (rglB) was found to produce an endonuclease activity (16). These restriction systems differ from the classically recognized ones since they require modified DNA as the substrate for their action rather than using modification for selfprotection.An additional methylation-specific restriction system of E. coli, Mrr, was described by Heitman and Model (20) and was shown to interfere with the maintenance of certain N6-adenine methylases. The HhaII and PstI N6-adenine methylase genes, when maintained in several E. coli K-12 strains, produced DNA damage as evidenced by induction of the SOS DNA repair response (20). They also demonstrated that several cytosine methylases induced the SOS response. Transposon insertion mapping and Southern blotting were used to position mrr on the E. coli chromosome at 98.5 min.

This report describes the application of reference strand-mediated conformational analysis (RSCA), a novel DNA typing technique, for the identification of clinically significant fungal pathogens. RSCA is a heteroduplex-based conformational method which relies on detecting differences in the DNA conformation of heteroduplexes generated in this study by the annealing of different fungal 18S rRNA amplicons to a common fluorescent-labelled reference (FLR). These heteroduplexes are then observed with laser-based instrumentation and computer software to detect differences in the DNA conformation reproducibly. This technique was shown to generate unique and reproducible profiles for the 18S rRNA gene sequences of a number of medically important fungi, distinguishing different Candida species (C. albicans, C. kefyr, C. dubliniensis, C. lusitaniae, C. guilliermondii, C. tropicalis, C. krusei, C. glabrata, C. sake and C. parapsilosis), and in some cases detecting single nucleotide differences between 18S rRNA sequences. The RSCA technique was further evaluated with 50 human clinical isolates of Candida spp., previously identified by culture techniques, and was shown to identify the isolates correctly. This technique displays enormous potential as an alternative to DNA sequence determination and has the potential to become an automated technique that can be implemented in the routine setting.