Nucleotide sequences of two genes encoding the small subunit of RUBISCO in Nicotiana sylvestris.

A genomic clone (NySS4) encoding sequences homologous to the small subunit (SSU) of RUBISCO was isolated from a N. sylvestris genomic library after a screening with a full-length SSU cDNA (1). This clone actually encodes two genes separated by a 200 bp intergenic region: NySS41 is a complete gene comprising 3 exons and 2 introns, while NySS42 is a truncated gene comprising only the 3' end of the third exon and a 3' noncoding region. NySS41 is highly homologous, but not identical to the TSSU 3 2 gene previously isolated from a N. tabacum genomic library (2). TSSU 3 2 is a pseudogene having a stop codon in its first exon which is transcribed at a very low level. NySS41 has no stop codon interrupting the reading frame, but it may be not transcribed or transcribed at a very low level since we did not find any corresponding cDNA in our extensive search for SSU cDNAs in N. sylvestris leaf extracts (1). On the other hand, NySS41 and NySS42 constitute the first cluster of SSU genes isolated in Nicotianeae.