In vitro mutagenesis of the LTA gene, encoding the A subunit of the Escherichia coli heat‐labile enterotoxin, has been used to obtain A subunits deficient in enzymic activity. One inactive A‐subunit mutant which contained two amino acid substitutions, was shown to associate with native B subunits to form a holotoxoid lacking toxin activity. A serine to phenylalanine mutation appears to be responsible for the loss of toxicity.

The nucleotide sequences of four variants of the Escherichia coli K88 antigen gene, K88ab1, K88ab2, K88ac, and K88ad, have been determined. The K88ab2 and K88ac sequences have not been reported previously. The K88ab1 sequence is very similar to that determined by other workers, but the K88ad sequence differs considerably from that described in a previous report. Comparison of the amino acid sequences inferred from the gene sequences revealed certain clusters of amino acid substitutions which have been correlated with areas of potential antigenicity in the mature proteins.

The heat‐labile enterotoxin genes from plasmid P307 have been cloned into pAT153. A comparison of the sequence of the LT‐A gene with that of the A subunit of cholera toxin shows extensive homology. There are a number of significant differences between the sequence of the LT‐A gene reported here and that published previously.

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SUMMARYIntegrated viral sequences and adjacent cellular sequences from the polyoma virus (Py)-transformed 53-Rat and 82-Rat cell lines which contain two and three partial early regions respectively, each in a single viral insert, have been molecularly cloned. Each of the cloned partial early regions have been subcloned and assessed with regard to their transcription, translation products (T antigens, TAgs) and biological activity including their transforming ability. The 53-Rat 5-3 kb EcoRI fragment is an intact Py EcoRI linear genome (derived from within the tandem duplicated sequences) which transforms rat cells with high efficiency and produces infectious virus when circularized and transfected into mouse cells. The 82-Rat cell line expresses three novel TAg species of 63K, 40K and 32K in addition to the Py middle and small TAgs. The 63K protein was found to be a truncated form of large TAg produced as the result of an addition/deletion in early region B sequences unique to large TAg. The 40K and 32K proteins are hybrid viral-cellular middle and large T Ags respectively, which are expressed from early region A that has been truncated by recombination with rat cellular DNA. Differences in the nuclear and cytoplasmic location of the different 82-Rat early region RNAs are due to RNA stability and/or transport from the nucleus to the cytoplasm most likely as a result of different cellular sequences at their 3' ends. Finally no common structural feature or sequence specificity was observed at the virushost DNA joins of the two cell lines.