FIG. 4.
A model for Sec incorporation. In the absence of an SBP2-SECIS complex, eEFSec does not have access to the ribosomal A-site (A). Arrow 1 serves to indicate SECIS binding by SBP2 as described in Fig. 2A. SECIS bound SBP2 is competent to recruit eEFSec (B) which results in a conformational in the SID that signals the ribosome to allow eEFSec to deliver Sec-tRNASec (arrow 2 and C). Arrow 3 indicates GTP hydrolysis by eEFSec and which results in tRNA release and dissociation of eEFSec from the ribosome (D). The fate of SBP2 after Sec incorporation is not known, as noted by the arrow and question mark in D.