This protein binds to 23S ribosomal RNA in the presence of protein L20 (By similarity). Required for karyogamy during female gametophyte development, when the two polar nuclei fuse to form the diploid central cell nucleus, and during double fertilization of the egg cell and the central cell (PubMed:16698901). Constitutively expressed in roots, stems, leaves, flowers, pistils and siliques. Failure of fusion of the polar nuclei during megagametogenesis and abnormal double fertilization; fails to undergo fusion of the outer nuclear membranes. Belongs to the bacterial ribosomal protein bL21 family. The predicted gene At4g30930 has been split into 2 genes: At4g30930 and At4g30935. The predicted gene At4g30930 has been split into 2 genes: At4g30930 and At4g30935.