Molecular characterization and evolution of the cytosolic class II 17.0 kDa small heat-shock protein gene family from Picea abies (L.) Karst

Abstract

In order to investigate the pattern of small heat-shock protein (sHSP) expression in Norway spruce, seedlings were exposed to thermal stress. [ 35S]methionine in vivo labelling revealed a set of eight predominant sHSPs with molecular weights ranging from 16.0 to 27.0 kDa, comprising a 17.0 kDa cytosolic class II sHSP which is already predicted from a previously cloned cDNA sequence (Forest Genetics 4 (1997) 131). Western blot analysis showed no significant amounts of the 17.0 kDa sHSP in non-stressed vegetative tissues, and upon heat shock it accumulates to levels comparable to those constitutively found in embryo tissues from mature seeds. The Picea abies 17.0 kDa sHSP is encoded by a gene family. Six functional genes and two pseudogenes were obtained from PCR-based cloning, indicating a high degree of sequence conservation with nucleotide identities between 88 and 99%. There is evidence that the gene family is subjected to gene conversion, preferentially homogenizing the 5′ moieties of the genes. The genes code for two distinct polypeptides with molecular weights of 16910 and 16870 Da, the former contains a putative phosphorylation site RXXS. Unlike angiosperm sHSP genes, those from P. abies contain two introns, located in the 5′ un-translated and coding region, respectively. Homologous introns exist in sHSP genes from Picea glauca, and Funaria hygrometrica, suggesting the presence of introns as a retained primitive condition of plant sHSP gene evolution.