These reports have suggested that {HEAD ~ the SIR2 proteins} are either histone ADP ribosyltransferases , or {TAIL ~ histone/protein} deacetylases , or both  
are
These reports have suggested that { HEAD ~ the SIR2 proteins } are either histone ADP ribosyltransferases , or { TAIL ~ histone / protein } deacetylases , or both
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH