Study of Structural Features of 5S rDNA Non-Transcribed Spacers of Citrus sinensis and C. reticulata

Orange (Citrus sinensis) and mandarin (C. reticulata) are economically important agricultural plants grown in many countries of the world, including southern Russia. In this regard, their genetic studies are widely carried out, including both sequencing of individual loci and whole genome sequencing. Some of the most important loci in a genome are the 5S ribosomal RNA gene arrays, which are tandem repeats. Their monomers consist of a conservative 120-nucleotide coding part and a non-transcribed spacer (NTS), which often has different length and sequence in different species. In this work, eight NTSs of orange (Hamlin variety) and 16 NTSs of mandarin (three willow-leaved mandarin as well as eight Dwarf Unshiu, four Tiahara, and one Unshiu varieties) were sequenced and studied. The obtained NTSs had different lengths and were divided into two classes: NTS_218 (217–221 bp long) and NTS_381 (381 bp long). The internal structure of NTSs of both classes was studied, including such characteristics as the sequence of the 16–17-bp start region involved in transcription termination and polyadenylation, the presence of poly-T and poly-G motifs, TATA-like motifs at position –25 to –30 bp from the end of NTS, the presence of microsatellite motifs, etc. The obtained results expand the theoretical understanding of the NTS nature, knowledge of the NTS structure in closely related species, and can be used in the analysis of Citrus hybrids and varieties bred with the participation of orange and mandarin.