The intergenic spacer (IGS) of the ribosomal DNA spans the region from the 3' end of the 25S rRNA gene to the 5' end of the 18S rRNA gene [12,14].Here we report the sequence of the IGS of tomato (Lycopersicon esculentum) cv. Rutgers. High molecular weight DNA was isolated [ 8 ], the rDNA was enriched in a CsCl-actinomycin D gradient [ 11] and used for restriction analysis and cloning. Unlike in most of the plants investigated [ 12,14] no length heterogeneity could be detected in the tomato rDNA repeating units. A 3.6 kb Eco RI and a 1.0 kb Barn HI fragment, comprising the entire IGS and the flanking coding regions, were ligated to pBR322 vectors and then transferred into competent HB101 cells. After sub-cloning sequence analysis according to Chen and Seeburg [3] was performed in an overlapping manner using dGTP and in addition dITP to avoid band compressions.The entire sequence of the IGS of tomato ( Fig. 1) is 3253 bp long with a G + C content of 53.7~o. The end points of the flanking coding regions were inferred from sequence comparison and are not shown in the figure.Examination of the sequence revealed the presence of two different tandemly arranged repetitive elements denoted RE I and RE II. As indicated in Fig. 1, RE I is present eight times (I/i-I/8) with an average length of 53 bp and a G + C content of 63.6~o. RE II is present six times (II/1-II/6) with an average length of 141 bp and a G + C content of 63.9~o. The two different repeats are separated from each other by an extremely ATrich region (A + T content 66~o). The putative transcription initiation region with the TATA and G G G G G boxes [12,17] is located within this AT-rich region directly in front of the RE II repeats between nt 1563 and nt 1575. At present the IGS sequences of maize [13, 17], rye [1], cucumber [6], radish [4], wheat [2] and mung bean [7] are known. Their comparison shows that they widely diverge in length, nucleotide sequence and organization which has greatly hampered the understanding of the presumed identical regulatory functions of these regions for transcription [ 1,5 ].Tomato is a widely used host plant for various viroids including potato spindle tuber viroid (PSTVd) which accumulates in the nucleoli of infected tomato cells [ 16]. Since the transcription and processing of rRNA takes place in the nucleolus [10] it has been surmised [15] that viroids could exert their pathogenicity via interaction with the IGS. When we compared the PSTVd sequence with the tomato IGS sequence we found many short stretches of limited similarity along the entire IGS chain. However, in all RE II repeats we detected a stretch of 25 nt out of which 19nt are identical with nt60-83 [9] of the The nucleotide sequence data reported will appear in the EMBL, GenBank and DDBJ Nucleotide Sequence Databases under the accession number X14639.
Two receptor binding variants of the influenza virus A/Tübingen/12/85 (H1N1) were separated by their different plaque formation in MDCK cells. Hemagglutination of variant I was restricted to red blood cells of guinea pigs, whereas variant II also hemagglutinated chicken cells. The variants differed also in their ability to bind to alpha 2,6-linked sialic acid. Evidence is presented that this difference is determined by a complex carbohydrate side chain at asparagine131 near the receptor binding site which is absent in variant II. With both variants, the arginine found at the cleavage site of all other human isolates analyzed so far was replaced by lysine.
The gene of a cytoplasmic 18 S ribosomal RNA (18 S rDNA) of the dicotyledonous plant tomato (Lycopersicon esculentum) cv. Rentita has been cloned, and its complete primary structure has been determined. The tomato 18 S rDNA is 1805 bp long with a G + C content of 49.6%. Its sequence exhibits 94%-96% positional identity when it is colinearly aligned with the previously reported sequences of the 17-18 S rDNAs of the dicot soybean and the monocots maize and rice. A model of the secondary structure of the 18 S rRNA of angiosperms is presented and its genera-specific structural features are compared with a current eukaryotic 18 S rRNA consensus model.
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