Selenium is an essential micronutrient that is required for various redox processes and that constitutes the 21st naurally occuring amino acid: selenocystein. [1]

Selenoproteins are selenocysteine-containing proteins that have been identified throughout all 3 kingdoms of life (bacteria, archaea and eukaryota). Comparison of various animal species shows that some organisms lost the selenocysteine insertion machinery during evolution. Therefore, some selenoproteins were lost, whilst others emerged during evolutionary history.

Selenocysteine is an aminoacid encoded by UGA. This codon is currently used to terminate protein synthesis. Nonetheless, in eukaryots, a selenocysteine is inserted when a stem-loop structure (known as the selenocysteine insertion sequence or SECIS element) is located in the 3’ untranslated regions of selenoprotein genes.

Whereas a variety of organisms have been analyzed for selenoprotein occurrence, a comprehensive survey of the vertebrate or the mammalian selenoproteomes has still not been done. [2]

Selenoproteins’s distinctive feature is the fact that they contain what is known as the 21st aminoacid: the selenocystein (Sec). This residue is encoded by the UGA codon (usually a STOP codon) that is present in the open reading frame of the gene, and it is decoded as Sec during the synthesis of ribosomal proteins thanks to specific mechanisms based on the classic translational apparatus. The main signal that will direct the recodification of the UGA codon is a secondary structure that is presented exclusively in the transcripts of selenoproteins, known as the SECIS element.

Sec is unique among other amino acids in that it is the only known amino acid in eukaryotes which biosynthesis occurs on its own tRNA, which is called tRNA[Ser]Sec. This tRNA is aminoacylated with a serine in a reaction catalyzed by the enzime seryl-tRNA synthetase (SerS) to generate the base structure for the Sec biosynthesis. Next, there is a phosphorylation of the serine by the enzyme phosphoseryl-tRNA kinase (PSTK). Selenium is in turn phosphorylated by selenophosphate synthetase 2 (SPS2), and then is added to the already phosphorylated serine. The enzyme Sec syntethase (SecS) then produces the Sec residue on this structure, forming a new structure called Sec-tRNA[Ser]Sec.[3]

Fig 1: Mechanism of Sec biosynthesis in eukaryotes. It is shown how phosphoseryl-tRNA kinase (PSTK) provides
the phosphorylated intermediate PSer-tRNA[Ser]Sec serving as a substrate for SecS. (Lavunskyy VM et al., 2014).

In order to incorporate the Sec-tRNA[Ser]Sec into the ribosome, it must be recognized by a specific elongation factor called eEFsec. This elongation factor does not directly interact with the SECIS element, but it does so through the SECIS binding protein known as SBP2. In addition, other factors such as ribosomal protein L30 (also known as eIF4a3) participate in the process of inclusion of the Sec in response to the presence of the UGA codon in the gene reading frame.[3]

Fig 2: Mechanism of Sec insertion in eukaryotes. The figure shows known factors that are required
for Sec incorporation into proteins in response to the UGA codon (Lavunskyy VM et al., 2014).

21 selenoproteins have been identified in all vertebrates: GPx1-4, TR1, TR3, Dio1, Dio2, Dio3, SelH, SelI, SelK, SelM, SelN, SelO, SelP, MsrB1 , SelS, SelT1, SelW1, Sep15. There are others that have been found only in certain lineages. These last ones suggest the existance of a dynamic process by which new selenoprotein genes were generated by duplication, while others were lost or replaced their seleocystein with a cysteine. Up to 28 different proteins have been found in the ancestral vertebrate selenoproteome.

The organism of study for our project is the birdParus major, commonly known as the great tit.

Taxonomy

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Class: Aves
• Order: Passeriformes
• Family: Paridae
• Genre: Parus
• Specie: P. major
• Subspecies: P. major major, P. major bargaensis, P. major kapustini, P. major excelsus, P. major corsus, P. major newtoni, P. major mallorcae, P. major ecki, P. major aphrodite, P. major niethammeri, P. major terraescanctae, P. major blanfordi, P. major karelini, P. major intermedius

Morphology

The great tit (Parus major) is the largest bird in the tit family, even though it does not exceed 14cm in length.

The plumage of its head is black with white cheeks, and its chest has a very characteristic black colour.It also has a great ability to hang from any vertical surface due to the strength and structure of its fingers (a feature that it’s shared with the rest of the tits).

Habitat

Parus major is a forest species that mainly lives in mature forests composed by deciduous species, specially trees of the Cupuliferae family (Quercus). It can also be found in coniferous forests although at lower densities.

The tree species don’t affect its distribution as much as they do in the case of other tits, as it tends to look for food in the bushes (closer to the ground) because of its lower flight speed. Its nesting requires holes, but it can use any kind of hollow: from holes in trees, walls or rocks to nest boxes, pots or wasted cans. Due to this great adaptability, it can also be found in non-forest environments, such as olive or almond fields, urban gardens, artificial trees, woods of rivers and streams, oak trees, etc. This is why its distribution is very wide in Eurasia.

Distribution

The species population appears to be increasing and it has an extremely large range of distribution (32,600,000 km2). Therefore, it does not approach the thresholds for Vulnerable under the range size criterion and it is considered to be at a least concern level (non-endangered species).

Etiology

The adults tend to be sedentary and defend a territory around the breeding area. In winter they tend to distribute themselves into flocks formed by two adult birds (a reproductive couple) and between 4 and 5 young birds that do not share any kinship with any of the other flock members. Half of these young individuals tend to be male and the other half female. The territory they occupy consists on a core or central nucleus (the best area: that supplies them with more resources and where the reproductive cuple can breed), a basic territory of the flock surrounding the core (that they defend against other flocks) and a pre-territorial zone (where they move through, enountering individuals from neighboring flocks).

Feeding

The great tit feeds mainly on insects, seeds and nuts.

Reproduction

The breeding season might oscillate depending on the weather conditions. It generally comprises the period of time from March to July, but sometimes it can be extended until August.

When the eggs hatch, between 6 and 7 chiks are born. These will start forming new flocks by August and will disperse, getting away from the place of birth. When the month of April comes, the adult reproductive couple of the flock will defend its core once again and start a new breeding.

The youngest birds will mate, reproduce among themselves and start defending territories within the basic zone.

In case of death of a member of the reproductive couple, one of the young birds will replace it and get access to the core of the territory. Therefore, access to a new breeding partner is guaranteed for adults by sharing the winter territory.

For further information you can visit the Wikipedia entry about Parus major.