The nucleotide sequence, expression, and evolution of one member of a multigene family encoding the small subunit of ribulose-1,5-bisphosphate carboxylase in soybean.

Abstract

We have examined the nuclear genes encoding the small subunit of ribulose-1,5-bisphosphate (RuBP) carboxylase from soybean. One member of this gene family, designated SRS1, has been isolated from a soybean DNA library constructed in the lambda vector Charon 4A. The complete nucleotide sequence and structure of this gene including its two introns and portions of the 5' and 3' flanking sequences were determined. The first exon encodes the entire transit peptide (55 amino acids) and the first 2 amino acids of the mature sequence. Based on analysis of the nucleotide sequence, we concluded that the precursor of the soybean small subunit consists of 178 amino acids. A gene-specific probe for SRS1 was used to show that this gene is transcribed and that steady-state levels of its transcript are strongly light regulated. S1 nuclease mapping was used to locate the potential start of transcription in the sequence and showed that the small subunit gene contains a cap site, TATA box, and -80 sequence, which match the consensus animal sequences. The mature SRS1 small subunit polypeptide of 123 amino acids contains 30 and 34 amino acid replacements relative to the previously determined amino acid sequences from pea and spinach, respectively. Southern blotting of restriction digests of soybean nuclear DNA and data on the complete structure of SRS1 suggest that a multigene family of at least 10 members encodes the RuBP carboxylase small subunit in soybean. Quantitative evolutionary comparison of the soybean small subunit sequence for SRS1 and the pea small subunit sequence suggests that these two genes diverged long before the divergence of pea and soybean.