SummaryWe have isolated and sequenced a set of deletions stimulated by DNA palindromes in Escherichia coli. All of the deletions are asymmetric with respect to the parental sequence and have occurred at short direct repeats. This is consistent with deletion by strand slippage during DNA replication. The orientation of the asymmetry in such deletion products is diagnostic of the direction of the strand slippage event. It is therefore also diagnostic of its occurrence on the leading or lagging strand of the replication fork when the direction of replication is known. In all cases in which the orientation of the asymmetry could be determined with respect to DNA replication, the products were consistent with a preference for deletion on the lagging strand of the fork. The data include replication slippage in three situations: on the chromosome of E. coli, in bacteriophage and in high-copy-number pUC-based plasmids.

Hepatitis C virus (HCV) seroconversion was detected by routine screening in a haemodialysis patient, Patient 1. Serological investigations were undertaken over the following 3 months to determine if further transmission to other patients on the unit had occurred. No additional cases were identified. Twenty-two haemodialysis patients known to have HCV infection were investigated using molecular epidemiological methods to determine if transmission between patients had occurred. HCV viraemia was demonstrated by polymerase chain reaction in 19 of 22 patients (86%). Genotyping showed that eight patients were infected with genotype 1, three with genotype 3 and eight, including Patient 1, with genotype 2. Phylogenetic analysis of viral sequences from the eight patients with genotype 2 revealed three, including Patient 1,with a novel subtype of HCV type 2, and revealed close similarity between viral sequences from patient 1 and those from one other patient, suggesting transmission. This was consistent with haemodialysis histories. Among other patients with genotype 2, there were two with subtype 2a and three others with three separate novel subtypes, as yet undesignated. With the exception of patient 1, all patients infected with novel subtypes were of Afro-Caribbean origin. The HCV prevalence among patients on the haemodialysis unit was high (14%), which may reflect the ethnicity of our haemodialysis population. This case emphasises the risk of nosocomial transmission and the importance of infection control procedures on haemodialysis units, and highlights the usefulness of molecular epidemiological techniques for the investigation of outbreaks of HCV infection.

SummaryFormation of araB-lacZ coding-sequence fusions is a key adaptive mutation system. Eighty-four independent arai3-lac2 fusions were sequenced. All fusions carried rearranged MuR linker sequences between the araB and lacZ domains indicating that they arose from the standard intermediate of the well-characterited Mu DNA rearrangement process, the strand transfer complex (STC). Five non-standard araB-lacZ fusions isolated after indirect sib selection had novel structures containing back-to-back inverted MuR linkers. The observation that different isolation procedures gave rise to standard and non-standard fusions indicates that cellular physiology can influence late steps in the multi-step biochemical sequence leading to ara6-lacZ fusions. Each araB-lacZ fusion contained two novel DNA junctions. The MuR-lacZ junctions showed 'hot-spotting' according to established rules for Mu target selection. The araB-MuR and MuRMuR junctions all involved exchanges at regions of short sequence homology. More extensive homology between MuR and am5 sequences indicates potential STC isomerization into a resolvable four-way structure analogous to a Holliday junction. These results highlight the molecular complexity of araB-lacZ fusion formation, which may be thought of as a multi-step cell biological process rather than a unitary biochemical reaction.

Hepatitis C virus (HCV) seroconversion was detected by routine screening in a haemodialysis patient, Patient 1. Serological investigations were undertaken over the following 3 months to determine if further transmission to other patients on the unit had occurred. No additional cases were identified. Twenty-two haemodialysis patients known to have HCV infection were investigated using molecular epidemiological methods to determine if transmission between patients had occurred. HCV viraemia was demonstrated by polymerase chain reaction in 19 of 22 patients (86%). Genotyping showed that eight patients were infected with genotype 1, three with genotype 3 and eight, including Patient 1, with genotype 2. Phylogenetic analysis of viral sequences from the eight patients with genotype 2 revealed three, including Patient 1,with a novel subtype of HCV type 2, and revealed close similarity between viral sequences from patient 1 and those from one other patient, suggesting transmission. This was consistent with haemodialysis histories. Among other patients with genotype 2, there were two with subtype 2a and three others with three separate novel subtypes, as yet undesignated. With the exception of patient 1, all patients infected with novel subtypes were of Afro-Caribbean origin. The HCV prevalence among patients on the haemodialysis unit was high (14%), which may reflect the ethnicity of our haemodialysis population. This case emphasises the risk of nosocomial transmission and the importance of infection control procedures on haemodialysis units, and highlights the usefulness of molecular epidemiological techniques for the investigation of outbreaks of HCV infection.