The regulation of a mRNA encoding a shoot-specific polypeptide from developing pea seedlings was studied and compared to the regulation of mRNAs encoding two major light-induced nuclear-encoded polypeptides, the small subunit of the ribulose 1,5 biphosphate carboxylase (ssRuBPCase) and a polypeptide of the light-harvesting chlorophyll a/b complex (LHCP). By using cDNA clones as probes in Northern blottings of total cellular RNA it was found that both ssRuBPCase and LHCP mRNA could be induced in shoots by white and red light but to lower levels in roots and cotyledons. In contrast, the mRNA for the shoot-specific polypeptide was only found in shoots, and was present approximately two days after the start of germination. The shoot-specific mRNA sequence was predominantly found in stem tissue, irrespective of illumination, both in the young seedlings and adult plants. Only very low amounts could be detected in plumule and leaf. The shoot-specific sequence could also be detected in RNA isolated from developing shoots of another pea cultivar but not in those of other legumes and of cereals. The primary sequence of the complete coding portion and the deduced amino acid sequence of the mRNA encoding the shoot-specific polypeptide was determined. The observed codon usage is non-random and is consistent with data from other high plant genes. Possible polyadenylation signal sequences (AATAAG and AATAAT) were present at 55 and 124 bases 5' of the poly(A) tail. The polypeptide encoded by the shoot-specific mRNA consists of 196 amino acids with a calculated molecular weight of 21 898. It contains a four times reiterated highly conserved unit of 26 amino acids. The NH2-terminal end is highly hydrophobic and resembles a signal polypeptide.