Characterization of a Lhcb5 cDNA from Scots pine (Pinus sylvestris).

Despite extensive studies on the LHC proteins of PSI and PSII (for review, see Jansson, 1994), the detailed polypeptide composition of LHC I and LHC I1 has not been understood until recently. The nature of the Lhcb5 gene product has been one of the major uncertainties, since it has been identified both as the CP26 (or the equivalent complex LHCIIc) polypeptide and one of two CP29 polypeptides (Jansson et al., 1992; Morishige and Thomber, 1992). Lhcb5 genes have previously been characterized from tomato (Pichersky et al., 1991) and barley (Sorensen et al., 1992); here we present the characterization of a Lhcb5 cDNA clone from the gymnosperm Scots pine (Pinus sylvestris) (Table I). Using a tomato Lhcb5 cDNA (Pichersky et al., 1991) as probe, we isolated partia1 Lhcb5 cDNAs, which subsequently were used to find a full-length cDNA, encoding a precursor Lhcb5 protein of 302 amino aads. The transit peptide cleavage site is not known for this protein due to N-terminal blockage of the mature polypeptide (Pichersky et al., 1991). A tentative cleavage site after Se?', which would give rise to a mature protein with a molecular mass of 27.1 kD, has recently been suggested (Jansson, 1994). Southem blotting showed that Lhcb5 is a single-copy gene in Scots pine, named Lhcb5.1, according to Jansson et al. (1992). We also investigated the light regulation of the Scots pine Lhca and Lhcb genes by transferring dark-grown Scots pine seedlings to light and measuring the &NA levels corresponding to Lhcal, Lhca2, Lhca3, Lhca4, Lhcbl.1, Lhcbl.2, Lhcb2, and Lhcb5. We found that a11 genes had a low but detectable expression in dark-grown Scots pine seedlings and were strongly light induced. We did not attempt to quantitatively determine the level of induction of the genes, but we estimate it to be at least 10-fold. Forty-eight hours of light was sufficient to fully induce the genes; subsequent light treatment did not result in further induction. We found a general correspondence between Chl synthesis and expression of the Lhca and Lhcb genes, similar to what Canovas et al. (1993) reported for Pinus palustris seedlings. In conifers Chl synthesis, rather than light per se, seems to be a major determinant for LhcalLhcb gene regulation.