SVR4 (suppressor of variegation 4) and SVR4-like: two proteins with a role in proper organization of the chloroplast genetic machinery

SUPPRESSOR OF VARIEGATION 4 (SVR4, also called MRL7) and its homolog SVR4-like (also called MRL7-Like) were originally identified as important proteins for proper function of the chloroplast in Arabidopsis. Both are nuclear-encoded chloroplast-located proteins, and knockout mutants of either gene result in seedling lethality. Transmission electron microscopy analysis revealed that chloroplast development is arrested at an early developmental stage in both mutants. Accordingly, in the mutant plants severely decreased levels of photosynthetic pigments as well as subunits of the photosynthetic complexes could be detected. In absence of either of the two proteins chloroplast DNA organization was clearly affected. Immunological analysis revealed that SVR4 is a component of the transcriptionally active chromosome (TAC) from barley chloroplasts. Analyses of gene expression indicate that SVR4 and SVR4-like are required for proper function of the plastid transcriptional machinery. We propose that SVR4 and SVR4-like function as molecular chaperones ensuring proper organization of the nucleoids in chloroplasts.