As is the case for some other RNA viruses, the amino acid sequences of retroviral proteins change at an astonishing rate. For example, the proteases of the human immunodeficiency virus (HIV) and the visna lentivirus with which it is often compared are as different as the proteases of fungi and mammals, and those of the human type I leukemia virus are as different from HIV or visna as are the proteins of humans and bacteria. That the sequences of retrovirus proteins can be recognized as sharing common ancestry with non-retroviral proteins implies that the vastly accelerated change has begun only recently or occurs very sporadically. Only a scheme whereby exogenous retroviruses exist as short-lived bursts upon a backdrop of germline-encoded endogenous viruses is consistent with the sequence data. Retroviruses are related to many other reverse transcriptase-bearing entities present in the genomes of eukaryotes. They also have proteins that are homologous with those of some plant and animal DNA viruses, and their reverse transcriptase is recognizably similar to sequences found in the introns of some fungal mitochondria. Computer alignment of all these sequences allows an overall phylogeny to be constructed that chronicles the history of events leading to infectious retroviruses.

The ABC excision nuclease of Escherichia coli is an ATP-dependent DNA repair enzyme composed of three protein subunits, UvrA, UvrB and UvrC. The DNA sequences of all three genes have been reported. UvrA, the component that binds directly to the DNA, and UvrB, which attaches itself to the UvrA-DNA complex, both contain consensus sequences though to be diagnostic of ATP-binding sites, although the UvrC sequence does not. We now report that a computer analysis of the UvrA sequence has revealed an unusual series of internal duplications centering around putative metal-binding sites which may be involved in the interaction with DNA. We also find a strong evolutionary relationship to a family of prokaryotic membrane-associated active-transport proteins.

Cowpox virus infection in cats is rare and usually leads to cutaneous lesions alone. Pulmonary infection and pneumonia have been documented occasionally but all such cases described to date have been fatal. Although usually affecting the upper respiratory tract, feline herpesvirus can also induce pneumonia. The present report describes the case of a cat that recovered from a pneumonia in which both poxvirus and feline herpesvirus were demonstrated.

3 x 106 cells/I with 6% monocytes, 6% basophils, 60% eosinophils, 21% lymphocytes, and 7% polymorphonuclear leucocytes on differential count. The red cell count was 7-5 x 107/1. The pleural fluid protein concentration was 52 g/l, glucose 110 mg/dl (6 mmol/l) and lactate dehydrogenase 400 IU/l. The arterial blood oxygen tension was 82 mmHg (10-9 kPa) with the patient breathing air. Ventilation-perfusion lung scintigraphy showed a nonsegmental defect at the right base, which was interpreted as being unlikely to be due to pulmonary embolism. A tuberculin test gave a negative result. Pleural fluid culture was negative for viral and bacterial pathogens and cytological examination gave negative results. Blood, sputum, and urine cultures were negative for bacterial pathogens. The serum cytomegalovirus infection titre by indirect immunofluorescence was 1/512 in the acute phase and rose to 1:16 384 five weeks after the onset of the patient's illness, at which time he was symptom free and the chest radiograph showed that the pulmonary inflltrate and pleural fluid had cleared. A repeat test for antinuclear antibody was negative. The patient remains in good health after 18 months of follow up. DiscussionCytomegalovirus infection pneumonia is most commonly seen in the compromised host. The most common cause of viral pneumonia in immunocompromised patients is cytomegalovirus infection, which has a predilection for recipients of organ transplants.2 In this setting pulmonary manifestations range from peribronchovascular consolidation or miliary nodules to diffuse consolidation.3 Rare cases of unilateral consolidation4 have been described and there has been a single case report of necropsy proved cytomegalovirus infection pneumonia in a compromised patient presenting as a solitary pulmonary nodule.s Although radiological evidence of cytomegalovirus pneumonia in the normal host is unusual, the resolution of the abnormalities as the clinical features subsided, together with the rise in cytomegalovirus infection titre, makes cytomegalovirus pneumonia the most likely cause of the acute illness in our patient. The course of spontaneous cytomegalovirus infection in previously healthy adults is usually that of a mild illness characterised by malaise, fever, atypical lymphocytosis, abnormal results in liver function tests, and negative heterophil agglutination.' Exudative pharyngitis is typi-957 on 11 April 2019 by guest. Protected by copyright.