Summary. Factor V (FV) deficiency (parahaemophilia) is an autosomal recessive bleeding disorder with an incidence of 1:10 6 . We have studied a young girl with very mild bleeding symptoms and undetectable levels of plasma factor V antigen and activity (<0·3% and <1·6% of normal, respectively). Both parents showed plasma levels of factor V activity of about 50% of normal. Sequence analysis of the 5 0 -and 3 0 -untranslated, coding and adjacent regions of the factor V gene revealed the presence of a 4 bp deletion in exon 13. Subsequent screening of members of the family for the mutation showed that both parents were heterozygous for the mutation, that one healthy sister carried only normal alleles, and that the patient was homozygous for the mutated allele. The mutation introduced a frameshift and a novel premature stop codon in codon 1303, and would predict the synthesis of a truncated factor V molecule that lacks part of the B domain and the complete light chain. However, no factor V heavy chain could be detected in the plasma of the patient. Furthermore, factor V activity could not be detected in the patients' platelets. This is the first reported mutation in the factor V gene that predicts a type I quantitative factor V deficiency. Surprisingly, the patient, who is homozygous for the mutation, so far has only a very mild bleeding tendency.

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SummaryResistance to activated protein C (APC), which is associated with the FV Leiden mutation in the large majority of the cases, is the most common genetic risk factor for thrombosis. Several laboratory tests have been developed to detect the APC-resistance phenotype. The result of the APC-resistance test (APC-sensitivity ratio, APC-SR) usually correlates well with the FV Leiden genotype, but recently some discrepancies have been reported. Some thrombosis patients that are heterozygous for FV Leiden show an APC-SR usually found only in homozygotes for the defect. Some of those patients proved to be compound heterozygotes for the FV Leiden mutation and for a type I quantitative factor V deficiency. We have investigated a thrombosis patient characterized by an APC-SR that would predict homozygosity for FV Leiden. DNA analysis showed that he was heterozygous for the mutation. Sequencing analysis of genomic DNA revealed that the patient also is heterozygous for a G5509→A substitution in exon 16 of the factor V gene. This mutation interferes with the correct splicing of intron 16 and leads to the presence of a null allele, which corresponds to the “non-FV Leiden” allele. The conjunction of these two defects in the patient apparently leads to the same phenotype as observed in homozygotes for the FV Leiden mutation.

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Bacteriocin 28b production is induced by mitomycin in wild-type Serratia marcescens 2170 but not in Escherichia coli harboring the bacteriocin 28b structural gene (bss). Studies with a bss-lacZ transcriptional fusion showed that mitomycin increased the level of bss gene transcription in S. marcescens but not in the E. coli background. A S. marcescens Tn5 insertion mutant was obtained (S. marcescens 2170 reg::Tn5) whose bacteriocin 28b production and bss gene transcription were not increased by mitomycin treatment. Cloning and DNA sequencing of the mutated region showed that the Tn5 insertion was flanked by an SOS box sequence and three genes that are probably cotranscribed (regA, regB, and regC). These three genes had homology to phage holins, phage lysozymes, and the Ogr transcriptional activator of P2 and related bacteriophages, respectively. Recombinant plasmid containing this wild-type DNA region complemented the reg::Tn5 regulatory mutant. A transcriptional fusion between a 157-bp DNA fragment, containing the apparent SOS box upstream of the regA gene, and the cat gene showed increased chloramphenicol acetyltransferase activity upon mitomycin treatment. Upstream of the bss gene, a sequence similar to the consensus sequence proposed to bind Ogr protein was found, but no sequence similar to an SOS box was detected. Our results suggest that transcriptional induction of bacteriocin 28b upon mitomycin treatment is mediated by the regC gene whose own transcription would be LexA dependent.Serratia marcescens has been shown to produce bacteriocins upon induction with DNA-damaging agents (50). These bacteriocins have been classified into two groups (23): fraction 1 bacteriocins are active against Escherichia coli but not against S. marcescens, and fraction 2 bacteriocins are active against S. marcescens but not against E. coli (50). Bacteriocins belonging to fraction 1 are simple polypeptides that resemble colicins (50). Only colicin-like bacteriocins L and 28b have been studied in some detail. Bacteriocin L from S. marcescens JF246 has been isolated and characterized, and the effects of this bacteriocin on the incorporation of labelled leucine and thymidine and on the cellular levels of ATP in E. coli were similar to those produced by pore-forming colicins (17,18,40). On the other hand, the bacteriocin 28b structural gene (bss) has been cloned and sequenced, and the predicted amino acid sequence of the C-terminal part of this bacteriocin has been shown to have a high degree of similarity to the C-terminal domains of pore-forming colicins (56). The two bacteriocins are very closely related, if not the same, and similar bacteriocins are produced by most S. marcescens biotypes (21).Colicin production is induced by mitomycin and other DNAdamaging agents (38). Determinants for colicin production studied so far are encoded by either small high-copy-number colicinogenic plasmids (type I) or large low-copy-number colicinogenic plasmids (type II) (38). An important feature of these plasmids is that they confer on their host...