The 5' ends of Escherichia coli lac mRNA.

We identified the predominant 5' ends of an mRNA in Escherichia coli to the exact nucleotides. There are four such ends of lac mRNA in fully induced cells. About 70% of the molecules have the reported major in vitro end, A-A-U-U-G (at +1), which is located 38 nucleotides before the A-U-G translation start. Another 15% start with A-U-U-G at +2, and about 8% start with A-U-U-A-G at -52. A fourth class of molecules begin with either A-G, C-A-G, A-C-A-G, or a weak A-C-A-C-A-G (at +24), observed only once. The origins of this latter set (less than or equal to 10% of the total) are not known, but they could represent "ragged" ends of the mRNA when it is degraded to the beginning of the ribosome-protected region of the message. The A-U-U-A-G molecules are probably initiated from an upstream promoter whose position would coincide with the cAMP-CRP DNA binding site for the major promoter.