Functional Analysis of Sinorhizobium meliloti Genes Involved in Biotin Synthesis and Transport

Abstract

External biotin greatly stimulates bacterial growth and alfalfa root colonization by Sinorhizobium meliloti strain 1021. Several genes involved in responses to plant-derived biotin have been identified in this bacterium, but no genes required for biotin transport are known, and not all loci required for biotin synthesis have been assigned. Searches of the S. meliloti genome database in combination with complementation tests of Escherichia coli biotin auxotrophs indicate that biotin synthesis probably is limited in S. meliloti 1021 by the poor functioning or complete absence of several key genes. Although several open reading frames with significant similarities to genes required for synthesis of biotin in gram-positive and gram-negative bacteria were found, only bioB, bioF, and bioH were demonstrably functional in complementation tests with known E. coli mutants. No sequence or complementation evidence was found for bioA, bioC, bioD, or bioZ. In contrast to other microorganisms, the S. meliloti bioB and bioF genes are not localized in a biotin synthesis operon, but bioB is cotranscribed with two genes coding for ABC transporter-like proteins, designated here bioM and bioN. Mutations in bioM and bioN eliminated growth on alfalfa roots and reduced bacterial capacity to maintain normal intracellular levels of biotin. Taken together, these data suggest that S. meliloti normally grows on exogenous biotin using bioM and bioN to conserve biotin assimilated from external sources.Microorganisms from the gram-negative genera Rhizobium, Sinorhizobium, Bradyrhizobium, Mesorhizobium, and Azorhizobium, collectively termed rhizobia, are well known for their capacity to establish N 2 -fixing symbioses with legume plants (2). Although the molecular basis of rhizobial N 2 reduction is defined (17) and a foundation has been constructed for understanding other bacterial genes expressed in the plant (25), our knowledge of how rhizobia grow on plant roots is less complete. Good growth of S. meliloti strain 1021 (Rm1021) on alfalfa roots requires external biotin (33), often supplied by plant roots (27), which regulates a gene, bioS, that helps S. meliloti compete under such conditions (13, 34). Whether biotin is essential or simply stimulatory for rhizobial growth has been long debated (36-38), but clearly, cell densities of Rm1021 and many other rhizobia under biotin-limiting conditions are increased greatly by small amounts of biotin (8). Growing S. meliloti serially under biotin-limited conditions produces several physiological and metabolic changes, including the accumulation of polyhydroxybutyrate and a significant reduction in cell size (8,14). Biotin-dependent enzymes such as pyruvate carboxylase are also affected under biotin-limited conditions, and several tricarboxylic acid cycle auxiliary enzymes show decreased activities (4, 5).Biotin is formed in bacteria by a well-defined pathway ( Fig. 1) (6,7,15,16,23,26). Many microorganisms, including Mesorhizobium loti, have biotin synthesis genes organized in operons (1,18,19,21,2...

Biotin uptake in prokaryotes by solute transporters with an optional ATP-binding cassette-containing module

“…Based on these bioinformatic data, previous work on biotin uptake in rhizobia (8,9), and the distinct similarity of BioMN to the CbiOQ and NikOQ modules of prokaryotic cobalt and nickel transporters (10), we hypothesized that BioM, BioN, and BioY may encode a tripartite, modular biotin-uptake system. bioM genes encode standard ATPases with the typical Walker A, Q loop, signature (LSGGQ), Walker B, and H motifs (see ref.…”

“…Because the products of these two genes share distinct similarity with CbiO and CbiQ, which are components of prokaryotic cobalt transporters, it has been proposed that bioMN may encode a biotin transporter (8). Comparative genomic analysis suggested, however, that bioY, located adjacent to bioMN in S. meliloti, is more likely to encode this transporter.…”

“…In 2002, Entcheva et al (8) reported that mutations in bioM and bioN lead to reduced biotin uptake in Sinorhizobium meliloti. Because the products of these two genes share distinct similarity with CbiO and CbiQ, which are components of prokaryotic cobalt transporters, it has been proposed that bioMN may encode a biotin transporter (8).…”

Structure, Function, and Evolution of Bacterial ATP-Binding Cassette Systems

“…In Sinorhizobium meliloti, mutations in the bioM and bioN genes lead to reduced biotin uptake. These two genes share strong similarity with the cbiO and cbiQ genes described above (140). Disruption of the Rhizobium etli bioM gene results in a mutant that takes up biotin at a lower rate than that of the wild type.…”

An ABC-Type Cobalt Transport System Is Essential for Growth of Sinorhizobium melilotiat Trace Metal Concentrations

“…Recently, Co 2ϩ was observed to be required for survival of an S. meliloti mutant that lacks the phosphotransferase system enzyme Hpr (48). BioM (CbiO homolog) and BioN (CbiQ homolog) in S. meliloti have been reported to import biotin (18), and these along with BioY appear to represent an ECFtype biotin transporter. A putative ABC-type cobalamin transporter (BtuF [Smb20056], BtuC [Smb20057], and BtuD [Smb20058]) was identified on the basis of sequence similarity to the siderophore/heme/vitamin B 12 family and the presence of a B 12 riboswitch in the 5Ј-UTR (69).…”