Do plastid envelope membranes play a role in the expression of the plastidgenome?

A unique biochemical machinery is present within the two envelope membranes surrounding plastids (Joyard et al., Plant Physiol. 118 (1998) 715-723) th at reflects the stage of development of the plastid and the specific metabo lic requirements of the various tissues. Envelope membranes are the site fo r the synthesis and metabolism of specific lipids. They are also the site o f transport of metabolites, proteins and information between plastids and s urrounding cellular compartments. For instance, a complex machinery for the import of nuclear-encoded plastid proteins is rapidly being elucidated. Th e functional studies of plastid envelope membranes result in the characteri zation of an increasing number of envelope proteins with unexpected functio ns. For instance, recent experiments have demonstrated that envelope membra nes bind specifically to plastid genetic systems, the nucleoids surrounded by plastid ribosomes. At early stages of plastid differentiation, the inner envelope membrane contains a unique protein (named PEND protein) that bind s specifically to plastid DNA. This tight connection suggests that the PEND protein is at least involved in partitioning the plastid DNA to daughter p lastids during division. The PEND protein can also provide a physical suppo rt for replication and transcription. In addition, factors involved in the control of plastid protein synthesis can become associated to envelope memb ranes. This was shown for a protein homologous to the E. coli ribosome recy cling factor and for the stabilizing factors of some specific chloroplast m RNAs encoding thylakoid membrane proteins. In fact, the envelope membranes together with the plastid DNA are the two essential constituents of plastid s that confer identity to plastids and their interactions are becoming unco vered through molecular as well as cytological studies. In this review, we will focus on these recent observations (which are consistent with the endo symbiotic origin of plastids) and we discuss possible roles for the plastid envelope in the expression of plastid genome. (C) Societe francaise de bio chimie et biologie moleculaire / Elsevier, Paris.