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The fimbrial subunit genes of Bacteroides nodosus may be divided into two distinct classes, based on the sequence of the major subunit gene fimA (accompanying paper -- Mattick et al., 1991). The genetic organization of the fimbrial germ region in these two clams is also distinct. Upstream of fimA in both classes in opposite transcriptional orientation is the gene aroA which encodes amino acid biosynthetic enzyme 5-enolpyruvylshikimate-3-phosphate synthase. However, downstream of fimA the two classes are quite different until homology is restored at a bidirectional transcription termination signal separating the fimbrial operon from a gene clpB, which appears to encode the regulatory subunit of an ATP-dependent protease. Between aroA and clpB class I strains contain, apart from fimA, only one other germ (fimB). Sequence and polymorase chain reaction analyses indicate that fimB does not have a separate promoter but rather is co-transcribed with fimA at a level attenuated by the strength of the transcription termination signal in the intergenic region. In class II strains fimA is followed by a more extended region containing three genes, which appear to have the same transcriptional arrangement as fimB. The second of these genes (fimD) may represent a functional analogue of fimb although there is no close sequence homology. The first gene (fimC) has no obvious similarity to either fimB or fimD. Beyond fimD, at the 3′ end of the class II-specific region, is a variant fimbrial subunit germ (fimZ) which is virtually identical in serogroups D and H and which appears to represent a duplicate, possibly redundant, gene closely related to the progenitor of the more divergent structural subunit fimA gene found in these strains. Comparisons of the predicted fimZ... [ABSTRACT FROM AUTHOR]