''Ensifer meliloti'' (formerly ''Rhizobium meliloti'' and ''Sinorhizobium meliloti'') are an aerobic,

Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

, and

diazotroph Diazotrophs are organisms capable of nitrogen fixation, i.e. converting the relatively inert diatomic nitrogen (N2) in Earth's atmosphere into bioavailable compound forms such as ammonia. Diazotrophs are typically microorganisms such as bacteria ...

ic species of bacteria. ''S. meliloti'' are motile and possess a cluster of peritrichous

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

. ''S. meliloti'' fix atmospheric nitrogen into

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

for their legume hosts, such as

alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, ...

. ''S. meliloti'' forms a

symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

relationship with

legume Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consum ...

s from the genera '' Medicago'', '' Melilotus'' and '' Trigonella'', including the model legume '' Medicago truncatula''. This symbiosis promotes the development of a plant organ, termed a

root nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known ...

. Because soil often contains a limited amount of nitrogen for plant use, the symbiotic relationship between ''S. meliloti'' and their legume hosts has agricultural applications. These techniques reduce the need for inorganic nitrogenous

fertilizer A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Man ...

Symbiosis

Symbiosis between ''S. meliloti'' and its legume hosts begins when the plant secretes an array of betaines and

flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...

s into the

rhizosphere The rhizosphere is the narrow region of soil or Substrate (biology), substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Pore space in soil, Soil pores in the rhizosphere can ...

: 4,4′-dihydroxy-2′-methoxy chalcone, chrysoeriol, cynaroside, 4′,7-dihydroxyflavone, 6′′-O-malonyl ononin, liquiritigenin, luteolin, 3′,5-dimethoxyluteolin, 5-methoxyluteolin, medicarpin, stachydrine, and trigonelline. These compounds attract ''S. meliloti'' to the surface of the root hairs of the plant where the bacteria begin secreting nod factors. This initiates root hair curling. The

rhizobia Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. I ...

then penetrate the root hairs and proliferate to form an infection thread. Through the infection thread, the bacteria move toward the main root. The bacteria develop into bacteroids within newly formed root nodules and perform nitrogen fixation for the plant. A ''S. meliloti'' bacterium does not perform nitrogen fixation until it differentiates into a

endosymbiotic An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root ...

bacteroid. A bacteroid depends on the plant for survival. Leghemoglobin, produced by leguminous plants after colonization of ''S. meliloti,'' interacts with the free oxygen in the root nodule where the rhizobia reside. Rhizobia are contained within symbiosomes in the root nodules of leguminous plants. The leghemoglobin reduces the amount of free oxygen present. Oxygen disrupts the function of the nitrogenase enzyme in the rhizobia, which is responsible for nitrogen fixation.

Genome

The ''S. meliloti''

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

contains four genes coding for

flagellin Flagellins are a family of proteins present in flagellated bacteria which arrange themselves in a hollow cylinder to form the filament in a bacterial flagellum. Flagellin has a mass on average of about 40,000 daltons. Flagellins are the princi ...

. These include ''fliC1C2–fliC3C4''. The genome contains three replicons: a

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

(~3.7 megabases), a chromid (pSymB; ~1.7 megabases), and a

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...

(pSymA; ~1.4 megabases). Individual strains may possess additional, accessory plasmids. Five ''S. meliloti'' genomes have been sequenced to date: Rm1021, AK83, BL225C, Rm41, and SM11 with 1021 considered to be the

wild type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, " ...

. Indeterminate nodule symbiosis by ''S. meliloti'' is conferred by genes residing on pSymA.

DNA repair

The proteins encoded by ''E. meliloti'' genes ''uvrA'', ''uvrB'' and ''uvrC'' are employed in the repair of DNA damages by the process of

nucleotide excision repair Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. Intercalation (biochemistry), intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single ...

. ''E. meliloti'' is a desiccation tolerant bacterium. However, ''E. meliloti'' mutants defective in either genes ''uvrA'', ''uvrB'' or ''uvrC'' are sensitive to

desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...

, as well as to UV light. This finding indicates that the desiccation tolerance of wild-type ''E. meliloti'' depends on the repair of DNA damages that can be caused by desiccation.

Bacteriophage

Several

bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...

s that infect ''Sinorhizobium meliloti'' have been described:Systematic naming of bacteriophages is rarely followed in the scientific literature, and a variety of phages can share the same name. While there exists an RNA phage called ΦM12, which infects

enterobacteria Enterobacteriaceae is a large family (biology), family of Gram-negative bacteria. It includes over 30 genera and more than 100 species. Its classification above the level of Family (taxonomy), family is still a subject of debate, but one class ...

, it is not synonymous with the DNA phage ΦM12 listed here. The same may be true for other phages in this list. Within this list, two phages have independently been named ΦM5. Φ1, Φ1A, Φ2A, Φ3A, Φ4 (=ΦNM8), Φ5^t (=ΦNM3), Φ6 (=ΦNM4), Φ7 (=ΦNM9), Φ7a, Φ9 (=ΦCM2), and Φ11 (=ΦCM9), Φ12 (=ΦCM6), Φ13, Φ16, Φ16-3,
The full genome of this phage is available a
NCBI
/ref> Φ16a, Φ16B, Φ27, Φ32, Φ36, Φ38, Φ43, Φ70, Φ72, Φ111, Φ143, Φ145, Φ147, Φ151, Φ152, Φ160, Φ161, Φ166, Φ2011, ΦA3, ΦA8, ΦA161, ΦAL1, ΦCM1, ΦCM3, ΦCM4, ΦCM5, ΦCM7, ΦCM8, ΦCM20, ΦCM21, ΦDF2, Φf2D, ΦF4, ΦFAR, ΦFM1, ΦK1, ΦL1, Note that this article was reprinted in ''Plant and Soil'' (1971) 35 (1): 63—66, which is where the URL and doi direct to. ΦL3, ΦL5, ΦL7, ΦL10, ΦL20, ΦL21, ΦL29, ΦL31, ΦL32, ΦL53, ΦL54, ΦL55, ΦL56, ΦL57, ΦL60, ΦL61, ΦL62, ΦLO0, ΦLS5B, ΦM1, ΦM1, ΦM1-5, ΦM2, ΦM3, ΦM4, ΦM5, ΦM5 (=ΦF20), ΦM5N1, ΦM6, ΦM7, ΦM8, ΦM9, ΦM10, ΦM11, ΦM11S, ΦM12, ΦM14, ΦM14S, ΦM19, ΦM20S, ΦM23S, ΦM26S, ΦM27S, ΦMl, ΦMM1C, ΦMM1H, ΦMP1, ΦMP2, ΦMP3, ΦMP4, ΦN2, ΦN3, ΦN4, ΦN9, ΦNM1, ΦNM2, ΦNM6, ΦNM7, ΦP6, ΦP10, ΦP33, ΦP45, ΦPBC5,This phage has never been formally reported in the scientific literature. However, the full genomic sequence has been uploaded to NCBI, availabl
here
ΦRm108, ΦRmp26, ΦRmp36, ΦRmp38, ΦRmp46, ΦRmp50, ΦRmp52, ΦRmp61, ΦRmp64, ΦRmp67, ΦRmp79, ΦRmp80, ΦRmp85, ΦRmp86, ΦRmp88, ΦRmp90, ΦRmp145, ΦSP, ΦSSSS304, ΦSSSS305, ΦSSSS307, ΦSSSS308, and ΦT1. Of these, ΦM5, ΦM12, Φ16-3Φ16-3 Complete Genome
/ref> and ΦPBC5 have been sequenced. As of March 2020 the

International Committee on Taxonomy of Viruses The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the taxonomic classification of and the nomenclature for viruses. The ICTV develops a universal taxonomic scheme for viruses, and thus has the means to appropri ...

(ICTV) has accepted the following species in its Master Species List 2019.v1 (#35): * Realm: ''

Duplodnaviria ''Duplodnaviria'' is a realm of viruses that includes all double-stranded DNA viruses that encode the HK97 fold major capsid protein. The HK97 fold major capsid protein (HK97 MCP) is the primary component of the viral capsid, which stores ...

'', Kingdom: '' Heunggongvirae'', Phylum: '' Uroviricota'' :* Order: '' Caudovirales'', Family: '' Myoviridae'', Genus: '' Emdodecavirus'' (formerly ''M12virus'') ::* Species: ''Sinorhizobium virus M7'' (alias ΦM7) ::* Species: ''Sinorhizobium virus M12'' (alias DNA phage ΦM12, type species) ::* Species: ''Sinorhizobium virus N3'' (alias ΦN3)

References

External links