Phylogeny and expression of the secA gene from a chromophytic alga--implications for the evolution of plastids and sec-dependent protein translocation.

Abstract

In bacteria many periplasmatic proteins are exported via the sec-dependent pathway. A homologous apparatus was found to be involved in the transport of proteins across the thylakoid membrane in plastids. In the present study additional data on the phylogeny and expression of one of the genes essential in this process, secA, is presented. For the first time, transcriptional activity of secA in the plastid was detected. When secA is used as a phylogenetic marker for plastid evolution it demonstrates a large phylogenetic distance between chlorophytic and non-chlorophytic (i.e. rhodophytic) primary plastids. This distance could be explained by assuming polyphyly for major plastid lineages. Moreover, it was found that two types of secA genes may exist in plastids. Whether or not these are involved in different protein translocation processes is presently unknown. In an attempt to identify further candidates, i.e. non-photosynthesis-related proteins, for sec-dependent protein transport, an SbpA protein was detected in chromophytic plastids by the use of a peptide antibody.