(A) Arrangement of tmk and holB in various genomes. Nucleotides at the junction of the two genes are shown. In H. influenzae Rd and E. coli, the reading frame of holB is offset −2 relative to that of tmk; in all others shown, the relative reading frame of holB is offset −1. In P. multocida (http://www.cbc.umn.edu/ResearchProjects/AGAC/Pm/pmhome.html), the two genes are separated by 4 bp. GenBank accession numbers and sources are as follows: Rd, reference 17 E. coli, reference 5; P. multocida, contig 186, nucleotides 22421 to 23201; Yersinia pestis, accession no. AF065312 (11); Mycoplasma pneumoniae, accession no. AE000016 (22); and Mycoplasma fermentans, accession no. AF100324 (8). (B) Proposed scenario for origin of lav homologs in various organisms. The original form of the gene in H. influenzae is arbitrarily shown as Int1 lav but could have been either a lav or a las ancestor. nm, N. meningitidis; ng, N. gonorrhoeae; hi, H. influenzae; Hae, biotype aegyptius.